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For the vanilla cupcakes:
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Preheat oven 350u00b0F. Prepare cupcake tins with 24 paper cupcake liners. Set aside.
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In a medium to large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt. Add the buttermilk, shortening, and vanilla. Beat with an electric mixer on low speed for 30 seconds. Then turn mixer up to medium-high speed and beat for 2 minutes longer, scraping bowl occasionally. Add egg whites and beat for 2 more minutes, scraping bowl occasionally. Pour into cupcake liners and bake for about 22 minutes, or until toothpick comes out clean. These cupcakes should be very moist; if they aren't, try baking them for less minutes the next time. Let cupcakes rest in pan for 5 minutes before removing them to a cooling rack to cool completely.
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For the lemon curd:
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In a medium stainless steel bowl placed over a saucepan of simmering water (do not allow bowl to touch the water), whisk together the eggs, sugar, and lemon juice until blended. Cook, stirring constantly, to prevent it from curdling. When the mixture becomes thickened (like sour cream), or reaches 165u00b0F, remove it from the heat. This should take about 10 minutes. Cut the butter into small pieces and whisk into the mixture until the butter has melted. Add the lemon zest and let cool. The lemon curd will continue to thicken as it cools. Cover immediately, so a skin doesn't form, and refrigerate for up to 1 week. (Be sure to reserve 3/4 cup of the lemon curd for the buttercream!)
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For the lemon Swiss meringue buttercream:
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In the heatproof bowl of a stand mixer set over a pan of simmering water (do not allow bowl to touch water), combine the egg whites and sugar, whisking constantly until the mixture is warm to the touch (about 130u00b0F).
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Place the bowl on the stand mixer fitted with the whisk attachment. Beat the egg white mixture on high speed until stiff peaks form. Continue beating until mixture is fluffy and glossy and completely cool, about 6 minutes.
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Switch to the paddle attachment. With the mixture on medium-low speed, add the butter, a few tablespoons at a time, beating well after each addition. Beat on lowest speed to eliminate any air bubbles, about 2 minutes. Gently fold in lemon curd with a rubber spatula.
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To assemble the cupcakes:
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With a paring knife, cut a small cone shape of cake out of the middle of each cupcake. Be careful to not cut to the bottom of the cupcake. Fill each cupcake with lemon curd, level with the top of the cupcakes. Then prepare a pastry bag with a plain round tip, filling it with the buttercream. Starting at the outer edge of a cupcake top, swirl the buttercream around the cupcake, up to the center.
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Source: Cupcakes recipe comes from my sister's recipe box, originally from a friend of hers. I just barely adapted it. The lemon curd recipe is the one my mom has been using for years, originally from Joy of Baking.com. I doubled the lemon zest of the original recipe. The lemon Swiss meringue buttercream is slightly adapted from Martha Stewart's Baking Handbook.
