1. In a large bowl, beat butter and sugar until crumbly, about 2 minutes. Add egg, then egg white, beating well after each addition. Beat on high speed until light and fluffy. Beat in the buttermilk, water, vinegar and vanilla. Combine the flour, cocoa, baking soda and salt; beat into batter just until moistened. Stir in chocolate chips.
2. Fill muffin cups coated with cooking spray three-fourths full. Bake at 375° for 15-18 minutes or until a toothpick inserted in the muffin comes out clean. Cool for 5 minutes before removing from pans to wire racks. Yield: 14 cupcakes.