Preheat your oven to 350 degrees Fahrenheit. Grease two 8x4-inch bread pans with butter or cooking spray.

In a small bowl, peel the bananas and mash them well with a fork. Mix in the brown sugar, vanilla extract, and ground cinnamon until smooth.

In a large bowl, cream together the softened butter and sugar until light and fluffy, about 2 to 3 minutes. Add the eggs one by one, mixing well after each addition.

Sift the flour, baking soda, and salt together in another bowl. Gradually add this dry mixture into the creamed butter and sugar, mixing until just combined.

Stir in the sour cream and then the banana mixture until just combined, ensuring no dry flour remains.

In a separate bowl, combine the flour, brown sugar, and cold butter using a fork or fingers to create a crumbly texture.

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

Bake for 55-65 minutes or until a toothpick inserted in the center comes out clean.

Remove from the oven and let the banana bread cool in the pans for about 10 minutes, then transfer to a wire rack to cool completely.