Preheat oven to 350F. Line 12 muffin cups with paper liners. Combine flour, cocoa, baking powder, soda and salt. Beat the next six ingredients until combined, about 1 minute. Stir in flour mixture alternately with milk until just combined. Pour batter into muffin cups. Bake 22-25 minutes or until toothpick comes out clean. Remove cupcakes from pan; cool on wire rack 30 minutes. Sprinkle with sugar, if desired.