1. For the cupcakes: Preheat the oven to 350 degrees, and prepare your cupcake pan with your desired liners.
2. In the bowl of your standing mixer fitted with the whisk attachment, beat the egg whites until foamy.
3. Add the granulated sugar to the eggs with the mixer running and continue to whisk, at medium-high speed, until the mixture is thick, stiff, and glossy. Transfer the egg white mixture to another bowl, and clean out your standing mixer bowl thoroughly.
4. In your clean mixer bowl, combine the butter and brown sugar and mix on medium speed until light and fluffy, about 1 minute.
5. Add the egg yolks, one at a time, mixing well after each addition.
6. In a separate bowl, combine the flour, cocoa, and baking soda.
7. With the mixer speed on low, slowly add the flour mixture, coffee, and Kahlua alternately, ending with the flour mixture.
8. Stop the mixer, and gently fold in the egg whites into the batter using a rubber spatula, until well-combined.
9. Fill each prepared cupcake liner about two-thirds full.
10. Bake the cupcakes for about 20-25 minutes, depending on your oven, or until a cake tester inserted into the center comes out clean. Let cool completely.
11. For the Kahlua buttercream: In the bowl of your standing mixer fitted with the paddle attachment, combine the butter and powdered sugar on low speed until smooth.
12. Add the cocoa powder, Kahlua, and coffee, adding more powdered sugar if necessary. Set aside.
13. For the Bailey’s buttercream: In the bowl of your standing mixer fitted with the paddle attachment (or another bowl if you are getting tired of cleaning your mixer bowl out….), combine the butter and powdered sugar until smooth.
14. Add the vanilla extract and Bailey’s Irish Cream, mixing well until combined.
15. Pipe the buttercream (description of how-to above) onto the cupcakes and garnish with chocolate curls.