Preheat the oven to 350 degrees. Spray a 9 inch square baking pan with cooking spray.

In a large bowl cream together the butter and sugar. Add in the cocoa powder and mix well.

Add the egg, vanilla, and milk, and mix well to combine.

In a medium bowl combine the flour, baking powder, and salt.

Add the flour mixture to the butter mixture and mix well to combine.

Stir in the mashed bananas.

Fold in the walnuts.

Pour the batter into the prepared pan. Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.

Cool completely then dust with powdered sugar or drizzle with caramel sauce.