• Preheat oven to 350°F. Line a loaf pan with parchment paper or heavily grease with butter.
• To make the bread: In a large bowl, cream together 1/2 cup butter, 1/4 cup granulated sugar, and 1/4 cup golden brown sugar. Add 2 eggs, 3 large ripe bananas, 1 teaspoon vanilla, and 1/2 cup sour cream. Mix until well combined.
• In a small bowl, combine 1 1/2 cups flour, 1 teaspoon baking soda, 1 teaspoon cinnamon, and 1/2 teaspoon salt. Add large spoonfuls of the dry ingredients to the wet and stir just enough to fully incorporate all of the ingredients. Fold in the 1 cup walnut pieces.

To make the topping:

• Mix together the topping ingredients into a small bowl. Mix well to combine. 
  1/2 cup brown sugar, 1/2 cup flour, 1/4 cup butter, 1 teaspoon cinnamon, 1/3 cup chopped walnuts
• Pour batter into prepared pan. Drop crumb mixture over surface of batter. Cook in preheated oven for about 60 minutes or until a toothpick inserted into the middle comes out clean.