Instructions

• Preheat your oven to 350 degrees F and line an 8x8-inch square baking pan with parchment paper, allowing the paper to hang over the sides about 2 inches for easy removal.
• First brown the butter. Follow the instructions in the referenced post to learn how to brown butter. It's easy!
• Make the streusel. Combine the flour, brown sugar, and cinnamon in a bowl so it is evenly blended and there are no lumps of brown sugar. Add soft butter and rub it in evenly until it forms soft crumbs. Cover the bowl and set aside.
• Make the cinnamon sugar mixture. Combine brown sugar, cinnamon, and cocoa powder and set aside.
• Make the banana batter. Combine flour, baking powder, baking soda, and salt in a medium bowl and whisk to blend evenly. Add mashed banana to a large mixing bowl and whisk in brown sugar until smooth and glossy. Add egg, sour cream, cooled browned butter, and vanilla and whisk together until evenly combined. Add the dry ingredients and gently fold them in using a rubber spatula.
• Spread two-thirds of the batter evenly into the pan. Sprinkle the cinnamon sugar evenly over the top to cover it entirely. Dollop remaining batter on top and then gently spread it out evenly. Crumble the streusel on top in an even layer.
• Bake for 35-40 minutes until a toothpick inserted into the center comes out clean. Transfer the pan to a wire rack and let cool completely.
• Make the glaze. Whisk together the powdered sugar, milk, and cinnamon in a small bowl. Drizzle the icing on top of the slightly warm cake and enjoy!