Prepare the streusel: whisk together the dry ingredients. Add the cold cube butter to it and cut the butter into the dry ingredient with a pastry blender or just use the tip of your fingers to press the butter into flat pieces. The streusel should resemble wet sand. Refrigerate until ready to use.
Preheat the oven to 350°F. Line a muffin pan with cupcake liners.
Mash the banana with a potato masher in a large mixing bowl.
Add melted butter and eggs to the mashed bananas. Whisk together to combine.
Add brown sugar and beat until smooth.
Sift flour, baking soda, and baking powder into a small mixing bowl. Stir in the kosher salt. Add the flour mixture to the liquid mixture and fold until just combined. There should be a few streaks of dry flour in the batter.
Add the toasted and chopped walnuts, fold to distribute.
Divide the batter into the prepared muffin pan with a large cookie scoop. One full cookie scoop per muffin. Sprinkle on top 1 tablespoon of streusel.
Bake for 20-22 minutes until a toothpick inserted in the center of the loaf comes out clean with just a few crumbs attached.
Allow the muffins to cool in the pan for 5-10 minutes before removing them from the pan and let them cool on their sides on a kitchen towel to help preserve their dome tops.