In a medium bowl, mix the flour, cinnamon, salt, brown sugar and melted butter with a fork til it clumps together. Place in the fridge to harden while you prepare the banana bread.

Preheat your oven to 350 F and grease and line a 9×5 loaf pan with parchment paper. In a small bowl, stir together the cinnamon sugar mixture by mixing the sugars and cinnamon. Break up any brown sugar lumps. Set aside. 

In a medium sized bowl, mash the bananas with 1/4 cup of brown sugar. Set aside.

In another medium-sized bowl, sift the flour, cinnamon, baking soda and salt.

In a large bowl or the bowl of an electric mixer, beat the butter, granulated sugar and remaining brown sugar for 2 minutes until well combined. Mix in the eggs one at a time followed by the vanilla extract. Mix in the sour cream followed by the banana mixture.

Use a rubber spatula to fold in the dry ingredients. Mix just til well combined. Do not overmix.

Pour or spoon half of the batter into the prepared tin. Smooth it evenly with an offset spatula. Evenly sprinkle the cinnamon sugar over the batter. Spread the remainder of the batter over top. 

Take the streusel out of the fridge. Use a fork to break up the streusel into small and mostly large clumps. Evenly sprinkle the streusel over the banana bread. 

Bake for 50-60 minutes or until a toothpick comes out clean or with only a few crumbs. 

Set the pan on a rack for 15 minutes. Remove bread from pan and place it on a rack to cool completely before slicing.  Store in an air tight container at room temperature.