For the cupcakes:
¾ cup (1½ sticks) unsalted butter, at room temperature
1½ cups sugar
3 large eggs
2 cups all-purpose flour
1 tsp. baking powder
¼ tsp. salt
1 tbsp. espresso powder*
½ cup strong brewed coffee
¼ cup buttermilk
2 tbsp. coffee-flavored liqueur (or additional buttermilk)
For the frosting:
3 large egg whites
½ cup sugar
1 vanilla bean, split lengthwise
¾ cups (1½ sticks) unsalted butter, at room temperature
2 tsp. espresso powder
2 tsp. very hot water
Chocolate-covered espresso beans, for garnish
Preheat the oven to 350˚ F. Line cupcake pans with paper liners. In the bowl of an electric mixer, combine the butter and sugar. Beat on medium-high speed until light and fluffy, about 3 minutes. Blend in the eggs one at a time, mixing well after each addition. In a small bowl, combine the flour, baking powder, salt and espresso powder. Whisk to blend. In a liquid measuring cup, combine the coffee, buttermilk, and coffee liqueur. With the mixer on low speed, add in the dry ingredients alternating with the liquids, mixing each addition just until incorporated and beginning and ending with the dry ingredients.
Divide the batter between the prepared cupcake liners, filling each about ¾ full. Bake 18-20 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pans 5-10 minutes, then transfer to a wire rack and let cool completely.
To make the frosting, combine the egg whites and the sugar in a heatproof bowl set over a pot of simmering water. Scrape in the seeds from the vanilla bean pods. 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. (The bowl should be cool to the touch.)
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 thick and smooth again, about 3-5 minutes more (or longer – don’t worry, it will come together!) Remove about half of the frosting to a bowl and set aside. In a small bowl, combine the espresso powder and hot water and whisk to dissolve. Blend the espresso powder mixture into the frosting remaining in the mixing bowl until smooth and completely incorporated, scraping down the sides of the bowl as needed.
Fit a pastry bag with a large tip (I used a large, unlabeled closed star tip.) Fill one side of the pastry bag with the vanilla frosting, and then fill in the other side with the coffee 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. Garnish the frosted cupcakes with chocolate-covered espresso beans, if desired.
*Espresso powder is generally a specialty ingredient not available in most grocery stores. It is available in specialty stores such as Williams Sonoma, Sur La Table, etc. or from many online retailers.