Step 1 Preheat oven to 350-degrees. Lightly spray a 9-inch pie pan with cooking spray. In a large bowl, combine the sugar, margarine, egg, Butter Buds, and vanilla until well blended. Stir in the flour, cocoa, baking powder, and salt. Spread in the prepared pan. Bake 20 minutes.
Step 2 Cool on a wire rack. Just before serving, spoon the whipped topping decoratively around the edges of the pie. Garnish with banana slices and walnuts.
Step 3 Cut the pie into eight wedges and serve with nonfat frozen vanilla yogurt, if desired. (Note if using yogurt be sure to add the extra calories and carbo exchanges -- check the label for specifics).