Place the slightly softened sherbet and the slightly softened yogurt or ice cream in a medium-sized bowl, and blend with a spoon until well mixed and of spreadable consistency. Do not allow the mixture to get runny. Set aside.

Using a large serrated knife, slice the frozen cake into 3 equal layers.

Place the bottom layer of the cake on a serving plate, and spread about half of the sherbet mixture over the cake. Place the middle cake layer over the sherbet, and spread with the remaining sherbet mixture. Top with the third cake layer, and smooth the outside of the cake by running a knife around it.

Wrap aluminum foil snugly around the sides of the cake. Then wrap the entire cake, including the serving plate, with additional foil or with plastic wrap, sealing it well. Place in the freezer for 45 minutes, or until the sherbet mixture is again frozen and firm.

While the cake is in the freezer, prepare the Fat-Free Whipped Cream, adding the orange extract along with the vanilla extract called for in the recipe. (If you're using a commercial frozen whipped topping, allow the topping to thaw before stirring in the orange extract.)

About 5 to 10 minutes before serving, remove the cake from the freezer so that it can soften a bit. (The sherbet mixture should remain frozen.) Slice while still frozen, placing each portion on an individual serving plate. Top with a spoonful of the whipped cream, and serve immediately.