Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch square baking pan or line it with parchment paper.
Make the Crumble Topping: In a medium bowl, combine the flour, brown sugar, and cinnamon. Add the cold butter pieces and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs. Set aside.
Prepare the Cake Batter: In a large bowl, cream the softened butter and granulated sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract and mashed bananas.
Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Combine Wet and Dry Ingredients: Add the dry ingredients to the banana mixture, alternating with the sour cream, beginning and ending with the dry ingredients. Mix until just combined.
Assemble the Cake: Pour the batter into the prepared baking pan and spread it evenly. Sprinkle the crumble topping evenly over the batter.
Bake: Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
