Beat margarine at high speed with an electric mixer until creamy; gradually add 1/2 cup sugar, beating well (about 5 minutes). Combine flour, baking powder, and salt. Add to margarine mixture alternately with milk, beginning and ending with flour mixture. Stir in vanilla. Beat egg whites at high speed until foamy. Gradually add remaining 3T sugar, beating until stiff peaks form. Stir one-third of egg white mixture into batter; fold remaining egg white mixture into batter. Pour into an 8-inch round cake pan coated with cooking spray. Bake at 350F for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack 5 minutes; remove from pan, and cool completely on wire rack. Split cake in half horizontally; place bottom layer, cut side up, on a serving plate. Spread with Vanilla Cream Filling; top with remaining cake layer, cut side down. Spread Chocolate Glaze over top of cake. Chill 3 hours. VANILLA CREAM FILLING: Combine first 5 ingredients in a saucepan, stirring with a wire whisk until blended. Bring to a boil over medium heat, and cook 1 minute, stirring constantly. Stir in vanilla. Cover pan and chill until set. Makes 1 1/2 cups. CHOCOLATE GLAZE: Combine sugar, cocoa, and cornstarch in a small saucepan. Gradually add milk, stirring until well blended. Bring to a boil over medium heat, and cook 1 minute, stirring constantly. Stir in vanilla. Makes about 1/3 cup. Recipe by: Americas Best Recipes, Winter 1995, page 83 Posted to JEWISH-FOOD digest by Linda Shapiro on Nov 02, 1998,