Preheat oven to 350°.
Coat bottom of an 8-inch square baking pan with cooking spray.
Combine sugar and next 8 ingredients in a bowl.
Lightly spoon flour into dry measuring cups; level with a knife. Add flour and dark chocolate to cocoa mixture, stirring just until blended. Spread batter in prepared pan. Bake at 350° for 30 minutes. Cool on a wire rack.