Preheat oven to 350 degrees.
Spray 9 inch square baking pan with cooking spray.
In large mixer bowl, combine brown sugar, yogurt, egg whites, and lemon extract.
Beat on medium speed until well blended.
In small bowl, stir together flour, cocoa, baking powder, and baking soda.
Add gradually to sugar mixture, beating until well blended.
Stir in lemon peel.
Pour batter into prepared pan.
Bake 35 - 40 minutes or until toothpick inserted in center comes out clean.
Cool 15 minutes; remove from pan to wire rack.
Cool completely.
Sift powdered sugar over top.
Store covered at room temperature.