Mix the margarine, sugar, egg yolks, and vanilla in a bowl. Beat until creamy. Heat until it forms a syrup. Sift together flour, salt, and baking powder. Sift four or five times. Add milk. Beat egg whites and gradually add to flour mixture. Pour the brown sugar syrup into a large pan. Then pour in the flour mixture. Bake in a 350°F oven until fork comes out clean, 35 to 45 minutes.