Preheat the oven to 350 degrees. In a medium bowl, mash 1.5 cups of bananas (about 3 large or 4 medium bananas).
Combine the mashed bananas with the eggs, cocoa powder, brown sugar, vanilla extract, salt, baking soda, and baking powder. Whisk together until smooth. Add the milk and whisk again. Make sure there are no clumps of cocoa powder.
Stir the oats into the milk/chocolate/egg mixture. Coat the inside of an 8×8 inch casserole dish or large pie pan with non-stick spray. Pour the oat mixture into the pan. Slice the last banana and place the slices over top of the oat mixture (this is an optional garnish).
Place the oats in the oven and bake for about 45 minutes. Cut into 6 pieces and serve warm.