Mocha Meringue Buttercream
4 large egg whites
3 sticks butter softened (1.5 cups)
1 cup sugar
1/4 teaspoon cream of tartar
8 oz bittersweet (or semisweet if you prefer) best-quality chocolate
1 tablespoon espresso powder
Place chocolate in a double boiler over simmering water on low heat for approximately 5–10 minutes. When chocolate is nearly all melted, add espresso powder and stir until mixture is completely smooth and no pieces of chocolate remain. Remove from the heat and let cool to lukewarm 5–10 minutes.
Place egg whites and sugar in a bowl and blend. Put the bowl over a pan of simmering water and whisk constantly until the mixture is warm to touch. (A good test is to dab your fingertips into the mixture and rub them together. If you can't feel any sugar crystals, the mixture is ready.)
Beat on medium-high for about three or four minutes. When soft peaks form, add cream of tartar and increase speed to high. Whisk for about 8 minutes more until stiff peaks form. (You'll know the meringue is done when you place your hands on the sides of the bowl and it is no longer hot; it is room temperature or cool to touch.)
Switch to paddle attachment and turn the mixer to low. Add butter, 3 tablespoons at a time until all combined. Scrape down sides. (Don't worry if your meringue is beginning to look soupy; just keep the faith!)
Add cooled chocolate to the mixer and combine on low, scraping down sides if necessary. (Now, it looks really soupy!)
Increase mixer to high speed and beat for one minute. You'll hear the meringue slap the sides of the bowl, which means it is ready!