Preheat oven to 325 F (160C). In a bowl, use an electric mixer to beat margarine and sugar until light and creamy. Beat in flour, cocoa, baking powder and vanilla. In another bowl, beat egg whites until they are snowy white and hold their shape. With a rubber spatula, fold beaten whites into cocoa mixture until completely blended. Pour into a greased and floured 8 inch (20 cm) square baking pan. Bake 20 to 25 minutes. Brownies should be slightly underbaked in the middle and firm to the touch near the edges. Cool completely before cutting.