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.
Mix in the vanilla extract.
Stir in the mashed bananas and buttermilk until combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Fold in the chopped walnuts if using.
In a small bowl, prepare the brown sugar streusel by combining the brown sugar, 1/3 cup flour, and ground cinnamon.
Cut the cold butter into the streusel mixture using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Pour the banana bread batter into the prepared loaf pan and spread evenly.
Sprinkle the brown sugar streusel evenly over the top of the batter.
Bake for 55 to 65 minutes, or until a toothpick inserted into the center comes out clean.
Remove the banana bread from the oven and allow it to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing.