Preheat oven to 350 degrees. Grease a 5x9 pan with butter.

In a medium bowl, combine flour, baking soda and baking powder, and mix well.

In a large bowl, stir together melted butter and maple syrup. Add in egg, bananas, and milk, then mix well (a few banana lumps are okay!). Gradually add flour to wet ingredients, 1/2 cup at a time, stirring after each addition, until fully incorporated.

Pour batter into greased pan, and bake for 50 minutes, or until an inserted toothpick comes out clean.