Preheat oven to 350°. Beat egg whites until stiff. Beat in 1/4 cup of sugar. In another bowl, cream margarine; add salt, vanilla, and remaining sugar gradually. Add egg yolks and beat until creamy. Sift flour with baking powder. Add alternately to creamed mixture with milk. Fold in beaten egg whites. Coat the blueberries with 1 tablespoon of flour and then fold into mixture. Turn into 8x8 inch pan. Sprinkle cinnamon sugar on top. Bake in 350° oven for 50 minutes. Cool on rack. Wrap any leftovers and store in refrigerator. May be frozen.