If you don't want to use brownie mix use a combination of unsweetened cocoa powder, brown sugar and flour
Sprinkles for deocoration
2-3 cups powdered sugar
1-2 tbsp. milk
2 tsp. vanilla extract
1/2 cup dry brownie mix from a box*
1 cup butter softened
2 tsp. vanilla extract
3/4 cup sour cream or plain yogurt
1/2 cup oil or melted butter
3 eggs
2 medium sized ripe bananas mashed
1 box cake mix