1. Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners. In a bowl, sift flour, cocoa, baking soda and salt.
2. Using an electric mixer on high speed, beat egg whites in a clean, dry bowl until stiff peaks form. Rinse and dry beaters. In a separate bowl, beat butter and sugar until light. Add yolks, one at a time, beating well after each. Beat in vanilla. With mixer on medium-low speed, beat in one-third of flour mixture, then half of buttermilk. Repeat, ending with flour.
3. Fold in one-third of egg whites. Fold in remaining whites and chocolate chips.
4. Divide batter among muffin cups; bake until a toothpick inserted into center of a cupcake comes out clean, 18 to 20 minutes. Let cool in pan on a wire rack for 5 minutes, then remove cupcakes to rack to cool completely. Sift confectioners' sugar over cupcakes, if desired.