Add the brown sugar and cinnamon to a small bowl. Whisk until the mixture is smooth and clump-free. Set aside.

In a small bowl, whisk together the brown sugar, all-purpose flour, cinnamon, and a pinch of Kosher salt.

Drizzle the melted butter over the flour-sugar mixture. Using the tines of a fork, stir everything together until you have a thick crumble. Set aside.

Line an 8×8'' metal baking pan with parchment paper. Then, grease with neutral oil or baking spray. Preheat the oven to 350°F.

Add the all-purpose flour, cornstarch, baking powder, and Kosher salt to a small bowl. Whisk until smooth. Then, place the bananas on plate or in a shallow bowl. Use the tines of a fork to mash the bananas until they're very smooth. Set aside.

To make the brown butter, melt the butter in a light-colored pan over medium heat. After a few minutes, it will start to simmer and foam up. Stir constantly until the butter smells toasty and there are brown bits on the bottom of the pan, about 5 minutes. Immediately remove the pan from the heat and pour the butter into a large heat-proof mixing bowl. You should have a 1/2 cup of brown butter. Let it cool for about 5 minutes.

Pour the sugar and brown sugar into the brown butter. Whisk together until you have a sandy mixture. Then, add the vanilla extract and eggs, whisking rapidly until the mixture is pale and thick, about 2-3 minutes. Next, stir in the sour cream and mashed bananas until it's thoroughly mixed.

Whisk half of the flour mixture into the butter-sugar mixture, followed by half of the milk. Repeat with the remaining ingredients, stirring just until the batter is combined. Don't over-whisk or the cake will be dense!

Carefully spread about half of the batter in bottom of the prepared pan. Sprinkle the cinnamon-sugar mixture evenly over the batter. Then, carefully spread the remaining batter over the cinnamon-sugar. (This can get a little messy, but do your best! It doesn't have to be perfect). Use a skewer to gently swirl the batter and cinnamon-sugar (don't over-swirl). Crumble the streusel evenly over the top of the cake.

Bake the banana coffee cake, checking for doneness at 1 hour and up to 1 hour and 10 minutes. The cake is finished when it no longer jiggles and an inserted toothpick has just a few moist crumbs on it. Let the cake cool for 20-30 minutes before slicing. Enjoy!