I usually make my cakes from scratch but I keep a butter cake mix on hand just for this cake. It's one of my favorite desserts that my Aunt Joan makes.
In a bowl I pour the cake mix and ADD the following: a small box of instant pistachio pudding mix (reserve about 1/2 tablespoon for frosting), 1/4c of sour cream and a little green dye. I proceed to follow the directions on the back of the box and bake. While the cake is baking I open up a tub of cool whip and mix in the reserved dry pudding mix (1/2 TBSP) and set aside in fridge.
After the cake has baked and cooled I spread the greenish cool whip on top and its done. It's the kind of cake that you cannot stop eating once you start so be sure to have company over to help you eat it. My sister in law HATES pistachios but LOVES this cake.
I also make it for Christmas because it's green. I make it in a 9x13 pan and then cut it so it looks like a Christmas tree.
It's crowd pleaser. Enjoy!