Heat oven to 350. Spray two 8-inch round or one 13x9 cake pan with cooking spray.
Stir together flour, cocoa and baking soda.
In saucepan over low heat, melt lowfat margarine, stir in sugar. Remove from heat.
Add milk, vinegar and vanilla to mixture in saucepan; stir.
Add flour mixture; stir with whisk until well blended.
Pour evenly into prepared pans. Bake 20 minutes or until wooden pick comes out clean.Cake is moist, so a little stain on the toothpick is ok.
Cool 10 minutes; remove from pans to wire racks. Cool completely.
In small mixer bowl, stir together 1 envelope dream whip, 1/2 cup cold skim milk, 1 T cocoa, and 1/2 t. vanilla. Beat on high about 4 minutes until soft peaks form. This step takes forever. Don?t give up.
For variations on the frosting, leave out the cocoa; add 1/4 t. almond extract instead of vanilla; add 1 T very strong coffee for mocha frosting.
Spread one cake layer with half the frosting. Top with other cake layer, spread remaining frosting on top. Refrigerate 2 or 3 hours, until chilled.