Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan and set aside.
In a medium bowl, whisk together 1 1/2 cups flour, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas and sour cream until just combined.
Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Do not overmix.
Fold in the chopped walnuts if using.
Pour the batter into the prepared loaf pan and spread evenly.
To make the topping, combine 1/2 cup flour, brown sugar, and cinnamon in a small bowl.
Cut in the cold butter pieces with a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Sprinkle the crumb topping evenly over the banana bread batter.
Bake in the preheated oven for 55 to 65 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the bread to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing.