Preheat the oven to 350°F (175°C) and generously grease two 8x4-inch loaf pans.

Mash ripe bananas in a small bowl with brown sugar, vanilla extract, and ground cinnamon until smooth.

Cream softened butter and granulated sugar in a large bowl until light and fluffy. Beat in the eggs until incorporated.

Gradually sift in the all-purpose flour, baking soda, and salt into the wet mixture, stirring until just combined. Gently fold in the sour cream and banana mixture.

Prepare the crumb topping by mixing flour, brown sugar, and cinnamon. Cut in cold butter until resembling coarse crumbs.

Divide the batter between the two greased pans and sprinkle the crumb topping over each loaf.

Bake for 35-40 minutes or until a toothpick comes out clean with a few moist crumbs.

Cool in pans for 10 minutes, then transfer to a wire rack to cool completely before slicing.