Here are the results from my recent Gooey Butter Cake experiments:

Chocolate Malt Gooey Butter Cake (My New Favorite)

For Crust:
1 box cake mix, yellow or chocolate
4 oz Melted Butter
1 Egg

Combine above and press in to the bottom of a 9 x 13 Cake Pan

For Filling:
8 oz Cream Cheese
4 oz melted butter
2 Eggs
1 lb Powdered Sugar
4 Tbs Cocoa Powder, Hershey's Special Dark worked best of the ones I've
tried
4 Tbs Malted Milk Powder

Cream together cream chesse, butter & eggs until smooth. sift together
remaining ingredients and slowly add to the cream cheese mixture.
Pour over crust and bake for 30-40 Minutes at 350. The filling shouldn't
be firmly set, but should still show motion when shaken.