Cocoa Cupcakes

Preheat the oven to 350°F. Line a 12-cup muffin tin with paper tins. In a large bowl, sift flour, cocoa, baking soda, and salt.Using an electric mixer on high speed, beat the egg whites in a clean, dry bowl until stiff peaks form. Wash and dry the beaters. In a separate bowl, whisk together the butter and sugar until smooth. Add the egg yolks one at a time, beating thoroughly after each one. Whisk with vanilla. Using an electric mixer on medium-low speed, beat a third of the flour mixture, then half of the buttermilk. Repeat, finishing with a sprinkle of flour.Add a third of the egg whites. Add the remaining egg whites and chocolate chips.Spread the batter over the muffin tins; bake until a toothpick inserted in the middle of the muffin comes out clean, 18 to 20 minutes. Let cool in the pan on a wire rack for 5 minutes, then transfer the cupcakes to a wire rack to cool completely. If desired, sprinkle the cupcakes with powdered sugar.