1. Preheat your oven to 350°F. Grease a 9×5-inch loaf pan with butter or nonstick spray, then line it with parchment paper for easy removal.

2. In a small bowl, mix together the flour, brown sugar, and cinnamon for the topping. Add the cold butter pieces and use your fingers or a fork to work the butter into the dry ingredients until it forms pea-sized crumbs. Set aside in the fridge while you make the batter.

3. In a large bowl, mash the bananas with a fork until mostly smooth with some small chunks. Stir in the melted butter, then add the sugar, beaten egg, vanilla, and sour cream. Mix everything together until well combined and creamy.

4. In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon. Pour the dry ingredients into the wet ingredients and gently fold together with a spatula. Stir just until no flour streaks remain.

5. Pour the batter into your prepared loaf pan and spread it evenly. Sprinkle the cinnamon crumb topping generously over the entire surface, breaking up any large clumps. Bake for 55 to 65 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs. If the top browns too quickly, tent loosely with foil for the last 15 minutes.

6. Let the bread cool in the pan for 10 minutes, then lift it out using the parchment paper and transfer to a wire rack. Let it cool completely before slicing for the cleanest cuts.