Place margarine and chocolate in a microwave-safe bowl. Microwave at HIGH 1 minute, stirring after 30 seconds, or until melted. Add sugar and cocoa; stir well. (Batter will be stiff.) Add egg whites and egg; stir well.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and baking powder; add to margarine mixture, stirring until smooth.
Pour batter into a 9-inch square pan coated with cooking spray. Sprinkle with caramels; swirl caramels into batter using the tip of a knife. Bake at 325° for 30 minutes. Cool in pan on a wire rack.