Preheat oven to 350 degrees F.
To make the banana coffee cake, mix the flour, baking powder, baking soda, and salt together in a bowl. Whisk until combined and set aside.
In a large mixing bowl, add the bananas. Mash completely. Add in the eggs, sour cream, maple syrup, oil, milk, vanilla extract. Using an electric mixer or whisk, mix until fully combined.
Pour the flour mixture into the wet ingredients and stir until just mixed. It is ok if there are still some lumps.
To make the cinnamon sugar center and topping, combine the brown sugar, flour, cinnamon, and salt together. Add in the cold cubed butter. Using a pastry cutter (or fork), cut the butter into the cinnamon sugar mixture. Continue to mix until it resembles sand like texture.
Pour half of the batter into a 9 inch round pie dish or 9 inch square baking pan. Divide the cinnamon sugar mixture in half and evenly sprinkle over the batter. Top with remaining batter and spread out with a spatula. Sprinkle the remaining cinnamon sugar mixture on top.
Place in the oven and bake for 28-35 minutes, or until the batter is set and a toothpick comes out clean. Let cool slightly and enjoy!