1
Prepare the cake mix, bake, and cool according to package directions.
2
Make the lemon filling: in a medium-size heavy nonaluminum saucepan, whisk the lemon peel, lemon juice, eggs, egg yolks, and Splenda (2/3 cup) until well blended. Heat over low heat for about 5 minutes, stirring often, until hot. When the mixture begins to thicken, stir constantly. Cook gently (don't boil or it will curdle) for about 3 minutes, or until the filling thickly coats the back of a metal spoon. Strain the mixture through a fine sieve set over a medium-size bowl. Place a piece of plastic wrap on the surface to prevent a skin from forming; refrigerate for at least 2 hours. The filling will thicken as it cools.
3
Make the strawberry filling: put the chopped strawberries into a small heavy saucepan and mash them into a chunky puree. In a small bowl, mix the Splenda (1/2 cup), water and cornstarch until blended. Stir this mixture into the berries and cook over medium heat until boiling. Reduce the heat to low and simmer, stirring constantly, for 1 minute, or until thickened and glossy. Cool slightly, then place plastic wrap directly on the surface and refrigerate for at least 2 hours.
4
To assemble: using a long serrated knife, slice the cake horizontally into 3 layers. Put one layer on a flat serving plate and spread with strawberry filling. Top with second cake layer. Gently spread with the lemon filling. Top with the remaining cake layer.
5
Decorate with Cool Whip and sliced strawberries.
