Preheat oven to 180ºC (350ºF). Line a 8"x8" square pan with cooking paper.
- Whisk butter and sugar until creamy. Add egg and vanilla; beat until thoroughly combined. Mix in bananas.
- Add the flour, baking powder, baking soda, and salt. (Be sure to sift the baking soda so no lumps go into the batter.) Stir until it's just mixed.
- Divide batter in half. Add cocoa to one half. Add 1-2 T milk to make that batter the same consistency as the plain half. Spread in 8” square pan.
- Spread the rest of the batter on top of the chocolate layer.
- Bake for 25-30 minutes or until a toothpick comes out clean.
- Cool on a wire rack. This is even better the next day, and makes for a great breakfast.