Chocolate Swiss Buttercream Recipe

Name: Chocolate Swiss Buttercream Recipe
Total time: 40 min
Yield: 48
Ingredients: 6 ounces egg whites (2/3 cup; 170g), from 5 to 6 large eggs; 11 ounces toasted sugar , Belgian cassonade, or light brown sugar (about 1 2/3 cups; 310g); see note; 1 teaspoon (4g) Diamond Crystal kosher salt (plus more if needed); for iodized salt, use half as much by volume or the same weight; 1/4 teaspoon cream of tartar; 16 ounces unsalted butter (4 sticks; 455g), softened to about 65°F (18°C); 1 teaspoon (5g) vanilla extract (plus more if needed); 8 to 14 ounces finely chopped dark chocolate, between 70 and 77% cocoa solids (about 1 1/3 to 2 1/3 cups; 225 to 395g); 1 teaspoon instant espresso powder (optional), dissolved in a few drops of hot water