Lightly spray 9x13 pan with cooking spray. Preheat oven to 350.
Combine cake mix, 8T (1 stick) melted butter and 1 egg. Mix well and press into the bottom of prepared pan
Beat cream cheese until smooth. Add remaining 2 eggs and cocoa powder, mix til combined. With mixer on low add powdered sugar and mix well. slowly add remaining melted butter and vanilla. Beat til smooth.
Stir in nuts (optional) Pour over cake bottom. Bake 40-50 min. DO NOT over bake. Center should still be gooey when finished baking. Let cool before cutting