Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8 inch square pan.
Cream margarine and 12 packets sweetener until fluffy. Add (salt and) vanilla. Separate eggs and reserve the whites. Add egg yolks to the margarine mixture; beat until creamy.
Combine 1 1/2 cups flour and baking powder; add alternately with milk to egg yolk mixture. Coat berries with 1 tablespoon flour and add to batter.
In a separate bowl, beat whites until soft peaks form. Add 5 packets of sweetener, 1 at a time, and beat until stiff peaks form.
Fold egg whites into batter. Pour into prepared pan.
Bake for 50 minutes, or until cake tests done.