YELLOW CAKE:Place dry ingredients in mixer bowl and mix at low speed to blend well. Combine 3/4 cup water, oil, sweetener,vanilla and egg substitute and mix with fork to blend. Add margarine to flour mixture along with liquid mixture and mix with a spoon only until well blended.Spread evenly in a 9" square pan whick has been greased with margarine. Bake 30-35 minutes at 375 F or until cake tester comes out clean and the cake pulls away from the sides of the pan. Cool to room temperature and cut 4x4 to yield 16 equal servings. FLUFFY FROSTING: Combine sufgar, water, sweet'n'low egg whites and cream of tartar in top of a doubl boiler and beat at high spped for 1 minute. Set over simmering water in the bottom of the double boiler. Continue to beat at high speed for 4-5 minutes or until soft peaks form. Remove from heat. Add vanilla to frosting and continue to beat at high speed 1-2 minutes or until thick enough to spread on a cooled cake. This is enough frosting for a 2-layer or 9" square cake.