Beat sugar, eggs, and vanilla until creamy. Add flour and baking powder. Scald milk and margarine. Then add to flour mixture. Mix until well blended. Pour into a well greased and floured 11" x 15" pan. Bake @ 350 for 20-25 min. Remove from oven and immediately spread peanut butter on top. Refrigerate until cooled. Melt chocolate bar and spread on top.