Prepare and bake cake mix according to package directions for three 8 inch round pans. Cool cakes completely, then split each in half to make 6 layers.
For the Lemon Custard: In a large saucepan or double boiler combine sugar and cornstarch. Mix well, then stir in egg yolks, lemon zest and lemon juice. slowly whisk in the boiling water. Cook over medium heat, stirring constantly, until mixture is thickened. Remove from heat and stir in butter. Set aside to cool.
Place the bottom layer of cake on a serving plate. Spread with approximately 1/5 of the lemon custard. Repeat layers. Frost top and sides with Seven Minute Frosting (see footnote for link to recipe).