Preheat the oven to 350°F. Spray a 9X9-inch square pan with baking nonstick spray. Line the bottom and sides with parchment paper and spray again.
In a large bowl, mix the melted butter (make sure it is cooled to room temperature), brown sugar, sugar, vanilla, eggs, sour cream, and mashed bananas (mash these well before adding to the bowl). Use a whisk to mix until combined and smooth.
½ cup Unsalted butter, ½ cup Brown sugar, ¼ cup White granulated sugar, 2 Large eggs, ¼ cup Sour cream, 1 teaspoon Pure vanilla extract, 3 Medium over ripe bananas
Add the flour, baking soda, salt, and cinnamon. Mix this in until just combined.
2 cups All-purpose flour, 1 teaspoon Baking soda, ½ teaspoon Salt, 1 teaspoon Ground cinnamon
Pour half of the banana batter into the pan and smooth using a rubber spatula. Sprinkle the cinnamon filling evenly on top of the batter. Use all of it. Spread the rest of the banana batter on top of the filling.
Pull the cinnamon topping out of the fridge and use your hands to break up the big clumps before sprinkles on evenly on top of the batter. Use all of it.
Bake for 35-45 minutes until a toothpick inserted comes out clean.
Let the cake sit on the hot pan for 10 minutes. Transfer the cake to a cooling rack by holding on to the parchment paper on the sides to lift it out of the pan.