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1
Make the lime curd: Combine the granulated sugar, egg and egg yolk, lime juice and cornstarch in a small saucepan.
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2
Cook over medium-low heat, whisking, until thick enough to coat a spoon, 8 to 10 minutes.
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3
Remove from the heat and whisk in the butter, a little at a time, until smooth.
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4
Strain through a fine-mesh sieve into a bowl, then stir in the lime zest and salt.
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5
Set in a larger bowl of ice and stir until the curd is completely cool.
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6
Press plastic wrap directly onto the surface; refrigerate until ready to use.
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7
Make the cupcakes: Preheat the oven to 350 degrees F and line a muffin pan with 12 cupcake liners.
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8
Pulse the wafer cookies in a food processor until finely ground.
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9
Melt 2 tablespoons butter; add to the processor and pulse until the mixture comes together.
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10
Press about 1 tablespoon crumbs into each muffin cup.
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11
Sift the flour, baking powder, baking soda and salt into a medium bowl; set aside.
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12
Combine the remaining 10 tablespoons butter and the granulated sugar in a large bowl and beat with a mixer on medium-high speed until fluffy, 5 minutes.
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13
Reduce the speed to low; beat in the eggs, one at a time.
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14
Add the vanilla, lime zest and juice and beat until well combined.
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15
Add the flour mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour mixture.
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16
Increase the speed to medium high and mix until combined.
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17
Divide the batter among the prepared muffin cups, filling each about three-quarters of the way.
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18
Bake until a toothpick inserted into the centers comes out clean, 20 to 22 minutes.
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19
Remove to a rack; let cool completely.
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20
Make the frosting: Beat the butter in a large bowl with a mixer on medium-high speed until fluffy, 5 minutes.
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21
Reduce the speed to low and beat in the confectioners' sugar.
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22
Increase the speed to medium high; add the sour cream, lime juice and salt and beat until fluffy, 2 more minutes.
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23
Place the lime curd in a pastry bag fitted with a 1/4-inch tip.
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24
Insert the tip into the center of each cupcake and squeeze to fill.
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25
Frost the cupcakes, then pipe more lime curd on top.
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26
Top with lime slices and zest.
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27
Cook's Note: Key limes are smaller and more tart than regular limes.
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28
If you can't find them, look for bottled Key lime juice and use regular lime zest.
