Add the following to any yellow or white cake mix. Betty Crocker is good.

5 beaten eggs
2 pkgs. pistachio instant pudding
1/2 c. milk
1/2 c. oil
1/2 c. water
3 drops green food coloring

Bake in bundt pan at 350 degrees for 45 minutes. Cool slightly and mix 1/2 cup milk and 1 package pistachio pudding mix and blend into a small container of Cool Whip. Spread on cake. Refrigerate if not using right away.