Directions

1. Prepare cake mix according to package directions; pour into a greased 9-in. square baking pan. Bake at 350 degrees F for 18-20 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
2. For filling, combine the sugar, cornstarch and salt in a saucepan. Gradually stir in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from heat; stir in butter, lemon peel and food coloring if desired. Gradually stir in lemon juice. Cool completely.
3. For frosting, in a mixing bowl, beat butter until fluffy. Gradually add confectioners' sugar. Add lemon juice, peel and enough milk to achieve spreading consistency.
4. To assemble, cut cake in half. Trim outside edges. Place one half on a serving plate. Spread with the lemon filling. Top with remaining cake; spread with frosting.