Preheat oven to 425F (220°C) and line or grease a 1.5 pound loaf pan
Mix flour, brown sugar, cinnamon, and salt for streusel. Stir in melted butter until crumbly and set aside
In a large bowl, add mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and whisk until smooth
Mix in sour cream and vanilla until well combined
Add flour, baking soda, and salt, mixing just until combined
Pour half the batter into the loaf pan
Sprinkle ¼ cup streusel and all the cinnamon sugar evenly over the batter
Add remaining batter
Top with remaining streusel
Bake at 425°F (220°C) for 10 minutes
Without opening the oven, reduce temperature to 350°F (175°C) and continue baking for 45–55 minutes until a toothpick comes out with moist crumbs
If the streusel browns too quickly, loosely tent with foil during the last 15–20 minutes
Cool in pan 15–20 minutes, then transfer to a rack
