Blend the butter with the brown sugar and cinnamon using your fingers or a wooden spoon. Mix in the nuts.

Preheat the oven to 350°F. Brush an 8x8-inch square or a 9x2-inch round pan with melted butter or spray generously with cooking spray.

Beat the butter, sugar and maple syrup with an electric mixer. Add the eggs one at a time. Scrape the sides and bottom of the bowl. Mix in the bananas.

Sift the banana flour, baking soda and salt; add to the banana mixture, blending well. Stir in the walnuts. Spread into the prepared pan. Drop the topping in small dollops onto the banana batter.

Bake 35 to 45 minutes or until the coffee cake is golden brown and a toothpick inserted into the center comes out clean. Cool in the pan for 30 minutes.

Invert onto a plate, then invert again so the coffee cake is right side up on a serving plate.