Oven at 425ºF.  One 8 inch round cake pan sprayed with nonstick cooking spray. About 1 1/2 hours before serving or early in day: 1. In medium bowl with fork, mix baking mix, skim milk, butter buds and 3 tablespoons sugar until blended. 2. Spoon batter into pan, sprinkle with extra sugar. Bake 15 minutes or until top is golden and toothpick inserted in center comes out clean. Cool cake in pan on wire rack 10 minutes. Remove from pan. Cool completely. 3. Meanwhile in 2 quart saucepan, medium heat, cook rhubarb, cornstarch and 1/2 cup sugar, stirring constantly until mixture thickens and boils. Boil 1 minute, stir in strawberries, remove from heat. Set aside to cool. 4. When cake and strawberry mixture are both cool assemble: Cut cake horizontally in half with serrated knife. Place bottom of cake on plate. Spoon strawberry mixture over. Top with Cool Whip topping, spread to edge. Place top of cake on next. 5. Garnish with dollops of Cool Whip with a whole strawberry on each.