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

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

In a separate bowl, cream together softened butter and sugar until light and fluffy, then add eggs one at a time.

Sift in flour, baking soda, and salt into the butter mixture and mix until just combined. Fold in sour cream and banana mixture.

For the crumb topping, blend brown sugar, ground cinnamon, and flour, then cut in cold butter until crumbly.

Pour batter into prepared pans and sprinkle crumb topping evenly over the batter.

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

Let cool in pans for 10 minutes, then transfer to a wire rack to cool completely.