For the cupcakes:
2 2/3 cups all-purpose flour
2/3 cup unsweetened cocoa powder
2-3 tsp. espresso powder
2 tsp. baking powder
1 tsp. baking soda
½ tsp. salt
1 cup whole milk
1 cup strong brewed coffee
1 tsp. peppermint extract
1 cup (2 sticks) unsalted butter, at room temperature
1 cup granulated sugar
1 cup light brown sugar
2 large eggs
For the frosting:
6 large egg whites
1¾ plus 2 tbsp. granulated sugar
1½ cups (3 sticks) unsalted butter, at room temperature
4 tsp. peppermint extract
1-2 tsp. vanilla extract
To make the cupcakes, preheat the oven to 350° F. Line two cupcake pans with paper liners. In a medium bowl, combine the flour, cocoa powder, espresso powder, baking powder, baking soda and salt; stir together. Combine the milk, coffee and peppermint extract in a liquid measuring cup.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugars. Beat on medium-high speed until light and fluffy, 2-3 minutes, scraping down the sides of the bowl as needed. Blend in the eggs one at a time. Alternately mix in the dry and liquid ingredients, beginning and ending with the dry ingredients and mixing just until incorporated.
Divide the batter evenly between the prepared cupcake liners. Bake 18-20 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan 5-10 minutes, then transfer to a wire rack to cool completely.
To make the frosting, combine the egg whites and sugar in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 160° F and the sugar has dissolved. Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes. Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until smooth again, about 3-5 minutes more. Stir in the peppermint and vanilla extracts and mix just until incorporated.
If you would like the red and white swirled frosting effect, remove about half of the frosting to a bowl. With the remaining frosting in the mixing bowl, add red gel coloring and mix until you have achieved the desired shade. Fit a pastry bag with a large tip. Fill one side of the pastry bag with the white frosting, and then fill in the other side with the red frosting (this does not need to be exact – just enough so that both colors are included.) Pipe a test streak until you see both colors coming out of the tip. Frost cupcakes as desired, refilling the pastry bag in the same manner as needed.