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In a large bowl or the bowl of a stand mixer, cream the butter, sugar, and brown sugar for one minute. Scrape the sides of the bowl and then beat in the egg until pale, about 2 minutes. Add the buttermilk, vanilla, and bananas and stir just to combine. Add the flour, baking soda, baking powder, cinnamon, and salt, and stir on low just until combined. Grease a small baking dish (I use an 8""x4"" pan but you can substitute whatever you have) and spread in the butter. Use your fingers to crumble the brown butter topping evenly over top of the batter. Bake in the preheated oven until the cake has fluffed and a toothpick inserted into the center comes out clean, about 30-35 minutes depending on your pan. Allow to cool and then drizzle with the topping if desired. To prepare the topping, combine the powdered sugar, vanilla, and milk in a small bowl until smooth. Add a smidge more milk if the mixture is too thick. Drizzle over top of the cooled cake.", "For the drizzle (optional):