To make the filling, combine the cornstarch and sugar in a medium-sized saucepan, and stir to mix well. use a wire whisk to slowly whisk in the milk. Place over medium heat, and bring to a boil, stirring constantly. Continue to cook and stir for 1 minute, or until the mixture is thickened and bubbly. Remove the milk mixture from the heat, and whisk in the lemon juice and rind. Cover and chill for several hours or overnight to thicken. Just before assembling the cake, make the topping by gently folding the yogurt into the whipped topping. Set aside. To assemble the cake, use a bread knife to cut the cake lengthwise into 3 layers. Stir the filling. Place the bottom cake layer on a serving platter, and spread with half of the filling. Repeat with the second layer. Top with the third cake layer, and spread the topping over the top and sides of the cake. Chill the cake for 1 to 2 hours, slice, and serve. Makes dozen Servings POINTS: 3 PER SERVING: 174 Calories, 1.3 g Fat, 0.8 g Fiber, 0 mg Cholesterol, 4.9 g Protein, 238 mg Sodium RECIPE FROM: Fat-Free Holiday Recipes by Sandra Woodruff, RD