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1
Fill a medium saucepan with 1 to 2 inches of water and bring it to a simmer over high heat; reduce the heat to medium low and keep the water at a bare simmer.
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2
Place all of the ingredients except the butter in a large heatproof bowl and whisk to combine.
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3
Set the bowl over, but not touching, the simmering water and whisk constantly until the mixture thickens and is the consistency of mayonnaise, about 13 to 15 minutes.
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4
(Periodically check to make sure the water isnt boiling by removing the bowl from the saucepan using a potholder or dry towel.
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5
If the water is boiling, reduce the heat so the eggs dont curdle.)
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6
Remove the bowl from the simmering water.
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7
Immediately whisk in the butter 1 piece at a time, waiting until each piece is completely melted and incorporated before adding the next.
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8
Set a fine-mesh strainer over a large bowl.
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9
Strain the curd, pressing on the solids and scraping the extra curd clinging to the underside of the strainer into the bowl; discard the solids.
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10
Immediately press a sheet of plastic wrap directly onto the surface of the curd to prevent a skin from forming.
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11
Refrigerate until set and chilled, at least 3 hours.
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12
(The curd can be made and refrigerated up to 3 days ahead.)
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13
Meanwhile, make the candied zest, if using.
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14
Using a vegetable peeler and being careful to avoid the white pith, remove the zest from the lemons and limes in large strips.
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15
Very thinly slice the strips lengthwise.
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16
Place the zest in a medium saucepan and add enough cold water to cover it by 1 inch.
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17
Place over medium-high heat and bring to a boil.
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18
Drain through a strainer and set the zest aside in the strainer; reserve the saucepan (no need to wash it).
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19
Place 1 1/2 cups of the sugar and the measured water in the reserved saucepan and bring to a boil over medium-high heat, stirring to dissolve the sugar.
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20
Add the reserved zest (reserve the strainerno need to wash it) and stir to combine.
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21
Reduce the heat to medium low and simmer until the zest is translucent, about 30 minutes.
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22
Meanwhile, place the reserved strainer over a medium heatproof bowl.
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23
Pour the mixture through the strainer (reserve the syrup for another use) and set the zest aside to cool slightly, about 3 minutes.
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24
Place the remaining 1 cup of sugar in a medium bowl.
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25
Working in small batches, toss the warm zest in the sugar, coating the strips thoroughly.
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26
Shake off the excess sugar, transfer the zest to a baking sheet, and spread it into a single layer.
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27
Let cool completely, about 10 minutes, and set aside.
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28
(Once cooled and dry, the candied zest can be covered with plastic wrap and stored at room temperature for up to 5 days.)
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29
Heat the oven to 350 degrees F and arrange a rack in the middle.
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30
Coat 2 (8-inch) cake pans with butter and flour and tap out any excess flour; set aside.
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31
Whisk the measured flour, baking powder, and salt together in a medium bowl; set aside.
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32
Place 1 1/4 cups of the sugar and the measured butter in the bowl of a stand mixer fitted with a paddle attachment and beat on medium speed until the mixture is airy and light in color, about 5 minutes.
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33
Add the zest and vanilla and mix until incorporated, about 30 seconds.
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34
Stop the mixer and scrape down the paddle and the sides of the bowl.
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35
Add a third of the reserved flour mixture and turn the mixer to low speed, mixing until the flour is just incorporated.
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36
Add half of the milk and mix until just incorporated.
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37
Continue with the remaining flour mixture and milk, alternating between each, until all of the ingredients are incorporated and smooth.
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38
Transfer the mixture to a large bowl.
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39
Wash and dry the bowl of the stand mixer.
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40
Place the egg whites in the bowl, attach it to the stand mixer, and fit the mixer with the whisk attachment.
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41
Whisk the whites on high speed until medium peaks form, about 1 minute.
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42
Add the remaining 1/4 cup of sugar and continue whisking until stiff, glossy peaks form, about 45 seconds to 1 minute more.
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43
Stop the mixer.
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44
Using a rubber spatula, fold a quarter of the egg whites into the cake batter until evenly incorporated; gently fold in the remaining egg whites until just combined and no pockets of white remain.
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45
Divide the batter between the prepared cake pans and spread it into an even layer.
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46
Bake until the surface of the cakes springs back when pressed and a toothpick or cake tester inserted into the center comes out clean, about 30 minutes.
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47
Remove the cakes from the oven, place on wire racks, and let cool for 15 minutes.
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48
Run a knife around the perimeter of each and turn them out onto the racks to cool completely, at least 45 minutes.
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49
When ready to serve the cake, chill the bowl of a stand mixer and the whisk attachment in the freezer for 10 minutes.
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50
Add all of the frosting ingredients to the chilled bowl and whisk on high speed until medium peaks form, about 1 to 1 1/2 minutes.
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51
Using a serrated knife, trim a very thin layer from each cake top, just enough to even out the surface.
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52
Slice each cake in half horizontally so that you have 4 layers.
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53
Rewhisk the lemon-lime curd until smooth.
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54
Place a cake layer on an upside-down cake pan or serving plate.
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55
With a long knife or offset spatula, evenly spread a third of the lemon-lime curd (about 1 cup) over the top of the cake, leaving a 1/2-inch border.
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56
Top with a second cake layer and evenly spread another third of the lemon-lime curd over the top of the cake, leaving a 1/2-inch border.
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57
Top with a third cake layer and evenly spread the remaining lemon-lime curd over the top of the cake, leaving a 1/2-inch border.
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58
Top with the fourth and final cake layer.
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59
(If the layers start to slide, use your hands to line them back up, then place in the refrigerator for 30 minutes to firm up.)
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60
Using a clean long knife or offset spatula, coat the top and sides of the cake with the whipped cream frosting in a smooth, even layer.
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61
If using the candied zest, sprinkle half of it on top of the cake, leaving a 1-inch border.
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62
Using your hands, gently press the remaining zest into the bottom 1 inch of the cake sides.
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63
Serve immediately.
