If you like coffee and chocolate, then you'll love these Mocha Cupcakes! Moist, deeply chocolate-y cupcakes are topped with a silky Swiss buttercream that's spiked with rich espresso.
Place the egg whites and sugar in a large metal mixing bowl set over a pot of simmering water. Cook, whisking occasionally, until the mixture is smooth, thin, and translucent, and it no longer feels gritty when rubbed between the thumb and forefinger (about 10 minutes).
Remove the bowl from the heat and whisk on medium speed until fluffy and stiff, and no hint of warmth remains when you place your hand on the side of the bowl.
Add the butter, a tablespoon at a time, whipping after each addition. If the mixture seems runny, place it in the refrigerator for 10 to 20 minutes, then re-whip.
Fold in the melted chocolate.
Place the instant espresso, water, and vanilla in a small bowl and stir together.
Whip the liquid into the chocolate buttercream, then transfer the frosting to a piping bag fitted with a 1M tip.
Pipe the frosting onto the cooled cupcakes, and dust with cocoa powder.