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 the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas and sour cream until combined.
Gradually add the dry ingredients to the wet ingredients, stirring just until 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 coffee cake topping, combine 1/3 cup flour, brown sugar, and cinnamon in a small bowl.
Cut in the cold butter pieces using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Sprinkle the topping evenly over the banana bread batter.
Bake for 55 to 65 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.