Combine margarine and sugar and beat until fluffy. Sift together the flour, baking powder and salt and add with the milk to the butter mixture. (Alternate 1/2 cup of the flour mixture with 1/4 cup milk, beat well, then add some more.) Beat in the vanilla. Fold in the egg whites.
Pour mixture into a greased 8"x8" baking pan. Combine topping ingredients and sprinkle over the top. Bake at 375° until a toothpick or knife inserted into the center comes out clean. (About 35 minutes.) Serve warm or at room temperature.