Prepare cake mix according to package directions, using a greased 9-in. square baking pan.
Bake at 350° for 18-20 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
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 zest and food coloring if desired. Gradually stir in lemon juice. Cool completely.
For frosting, in a small bowl, beat butter until fluffy. Gradually add the confectioners' sugar, lemon juice, lemon zest and enough milk to achieve desired spreading consistency.
To assemble, cut cake in half lengthwise. Trim outside edges. Place one half on a serving platter. Top with the lemon filling. Top with remaining cake; spread with frosting.
Editor's Note: This recipe was tested with Jiffy cake mix.