Key Lime Cupcakes
- For the Lime curd:
- 1/2 cup granulated sugar
- 1 large egg plus 1 egg yolk
- 1/4 cup fresh Key Lime juice (from about 8 key limes)
- 2 tsp. cornstarch
- 4 Tbl. unsalted butter, cut into pieces
- 1 tsp. grated Key lime zest (from about 2 key limes)
- Pinch of salt
- For the cupcakes:
- 12 chocolate wafer cookies
- 12 Tbl. unsalted butter, at room temperature
- 1 1/2 cup all purpose flour
- 1/2 tsp. baking powder
- 1/4 tsp. baking soda
- 1/4 tsp. salt
- 3/4 cup granulated sugar
- 2 large eggs, at room temperature
- 1 tsp. vanilla extract
- 1 tsp. grated key lime zest
- 2 Tbl. fresh key lime juice (from about 4 key limes)
- 1/3 cup buttermilk
- For the frosting:
- 1 stick unsalted butter, at room temperature
- 3 1/4 cup confectioners' sugar
- 1/4 cup sour cream
- 1 Tbl. fresh key lime juice
- Pinch of salt
- Key lime slices and grated zest, for topping
Make the lime curd: Combine the granulated sugar, egg and egg yolk, lime juice and cornstarch in a small saucepan. Cook over medium-low heat, whisking, until thick enough to coat a spoon, 8 to 10 minutes. Remove from the heat and whisk in the butter, a little at a time, until smooth. Strain through a fine-mesh sieve into a bowl, then stir in the lime zest and salt. Set in a large bowl of ice and stir until the curd is completely cool. Press plastic wrap directly onto the surface; refrigerate until ready to use.
Make the cupcakes: Preheat the oven to 350 degrees and line a muffin pan with 12 cupcake liners. Pulse the wafer cookies in a food processor until finely ground. Melt 2 Tbl. butter; add to the processor and pulse until the mixture comes together. Press about 1 Tbl. crumbs into each muffin cup.
Sift the flour, baking powder, baking soda and salt into a medium bowl; set aside. Combine the remaining 10 Tbl. butter and the granulated sugar in a large bowl and beat with a mixer on medium-high speed until fluffy, 5 minutes. Reduce the speed to low; beat in the eggs, one at a time. Add the vanilla, lime zest and juice and beat until well combined. Add the flour mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour mixture. Increase the speed to medium-high and mix until combined.
Divide the batter among the prepared muffin cups, filling each about three-quarters of the way. Bake until a toothpick inserted into the center comes out clean, 20 to 22 minutes. Remove to a rack; let cool completely.
Make the frosting: beat the butter in a large bowl with a mixer on medium-high speed until fluffy, 5 minutes. Reduce the speed to low and beat in the confectioners' sugar. Increase the speed to medium high; add the sour cream, lime juice and salt and beat until fluffy, 2 more minutes.
Place the lime curd in a pastry bag fitted with a 1/4 inch tip. Insert tip into the center of each cupcake and squeeze to fill. Frost the cupcakes, then pipe more lime curd on top. Top with lime slices and zest.