For the Cupcakes:
- 2 1/4 cups all-purpose flour
- 1/2 cup unsweetened cocoa powder, sifted
- 1 1/2 teaspoons baking soda
- 3/4 cup coffee, at room temperature
- 3/4 cup Kahlua
- 3 large eggs, separated, at room temperature
- 3/4 cup granulated sugar
- 1/2 cup (1 stick, 8 tablespoons) unsalted butter, at room temperature
- 1 cup packed light brown sugar
For the Kahlua Buttercream:
- 3/4 cup (1 1/2 sticks, 12 tablespoons) unsalted butter, at room temperature
- 6 cups confectioners' sugar, sifted
- 6 tablespoons unsweetened cocoa powder, sifted
- 1/2 cup Kahlua
- 2 tablespoons coffee, at room temperature
For the cupcakes:
Preheat oven to 350 degrees F. Line cupcake pans with paper liners.
Whisk together the flour, cocoa, and baking soda in a medium bowl. Combine the coffee and Kahlua in a liquid measuring cup.
Add the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-low speed until foamy. With the mixer still running, gradually add the granulated sugar. Once incorporated, increase the mixer to medium-high speed and continue beating until stiff peaks form. The egg whites should be glossy, not dry.
Transfer the egg whites to another bowl and clean our the mixer bowl (or use a second bowl, or use a hand-held electric mixer). With the paddle attachment, cream the butter and brown sugar on medium speed until light and fluffy. Add the egg yolks, one at a time. With the mixer on low speed, alternatively add the flour and the coffee-Kahlua mixture, beginning and ending with the flour mixture. Use a rubber spatula to fold in egg whites.
Divide batter evenly between cupcake pans, filling each about 2/3 - 3/4 full. Bake for 18-20 minutes, or until the cupcakes spring back when lightly pressed and a toothpick inserted in the center comes out clean. Transfer the pan to a wire rack and let the cupcakes cool for 5 minutes then remove to rack to cool completely.
For the buttercream:
Beat the butter and powdered sugar in the bowl of a stand mixer fitted with the paddle attachment. Add the cocoa powder, Kahlua, and coffee and beat to incorporate. Once the frosting comes together, increase speed to medium-high and beat until smooth and fluffy. Spread or pipe with a decorative tip onto cooled cupcakes.