Melt margarine and chocolate in a large saucepan over medium heat. Stir in cocoa; cook 1 minute. Stir in sugar, and cook 1 minute (mixture will almost form a ball and be difficult to stir). Remove pan from heat; cool slightly. Gradually add warm chocolate mixture to egg whites and egg, stirring with a whisk until well-blended. Combine flour and baking powder; add flour mixture to chocolate mixture, stirring well.
Spoon batter into a 9-inch square baking pan coated with cooking spray. Bake at 325° for 30 minutes (do not overbake). Cool on a wire rack.