Prepare the cake mix using the egg, water, and lemon extract, following label directions. Pour into a greased and floured 20 cm round cake pan. (9 inch is too large and causes cake to be too flat.)

Bake at 180ºC or time indicated on the cake package, or until the center of the cake springs back when lightly touched. Cool cake in pan on rack for 10 minutes. Remove from pan and cool completely.

Prepare pudding mix with milk and vanilla and cook according to package directions. Cover with plastic wrap to prevent skin from forming on top and cool completely.

Split layer cake, fill with cooled vanilla pudding, put cake back together and then frost with enough chocolate frosting to cover the top. Store in refrigerator.