Melt margarine, blend in sugar and vanilla. Add eggs, beating well. Combine flour, cocoa, baking powder and salt. Gradually add to egg mixture until well blended. Spread in greased 9 x 13 inch baking pan. Bake at 350 degrees for 40-45 minutes. Cool and dust with powdered sugar.
Madagascar Vanilla (purchase at Chefs Catalog) Of all the vanilla's I've used this is my favorite. Once you've tried it you won't go back!