1 Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13 inch pan. Sift together the flour, cocoa, baking soda and baking powder. Set aside.
2 In a large bowl, cream together the butter, shortening and sugar until light and fluffy. Add the eggs one at a time, beating well with each addition, then add the vanilla. Add the flour mixture alternately with the milk. Mix to combine.
3 Pour batter into a 9×13 inch pan. Bake for 30 minutes in the preheated oven, or until a toothpick inserted into the cake comes out clean. Allow to cool.
4 Spread whipped topping over cooled cake and place sliced bananas on top. Store in refrigerator.