Preheat oven to 350°F. Grease or spray 4 mini bundt pans with nonstick spray (note - check spray for gluten.)
Beat egg whites in a glass or metal bowl until soft peaks form. I just tossed mine in the stand mixer on a medium speed while I prepared the rest of the batter.
Beat 1-1/2 cups confectioner's sugar and ½ cup butter together in a bowl until creamy, then add the egg yolks until fully blended.
Sift together flour and baking powder, and stir flour mixture into creamed butter mixture, alternating with cooled coffee until just mixed. Finally, fold egg whites into the batter. Pour batter into the prepared pans.
Bake for 25-35 minutes (or 50-60 minutes for a large pan) until the edges of the cakes pull away from sides and a toothpick inserted in the center comes out clean. Cool cakes for at least 30 minutes before turning out of pans onto serving plates.
Prepare the icing by beating 2 cups confectioners' sugar, ½ cup butter, and cocoa powder together in a bowl until smooth; beat 2 Tbsp. coffee in until icing gets creamy, and add additional coffee a teaspoon or two at a time until you reach desired consistency - a slightly thinner mixture than traditional frosting.
Add icing to piping bag and drizzle over cakes.
Recipe by Happy Hour Projects at https://happyhourprojects.com/gluten-free-recipe-mocha-cake/