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 buttermilk until just combined.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the chopped walnuts if using.
In a small bowl, combine the brown sugar, 1/4 cup flour, and cinnamon for the streusel. Cut in the cold butter pieces 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 from the oven and let the bread cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing.