How to Make It
Preheat the oven to 350° and line a 12-cup muffin tin with paper or foil liners. In a large bowl, mix the mashed bananas with the brown sugar, egg, milk, vanilla and all but 1 tablespoon of the melted butter. In a medium bowl, whisk the flour with the baking powder, salt and 1/2 teaspoon of the cinnamon. Add the dry ingredients to the banana mixture and whisk until the batter is evenly moistened. Stir in 3/4 cup of the chopped walnuts. Spoon the batter into the prepared muffins cups.
In a small bowl, mix the granulated sugar with the remaining ¼ teaspoon of ground cinnamon and ¼ cup of chopped walnuts. Add the remaining 1 tablespoon of melted butter and mix gently until moist crumbs form. Sprinkle each muffin with some of the streusel.
Bake the muffins for about 25 minutes, until a toothpick inserted in the centers comes out clean with a few moist crumbs attached. Let the muffins cool in the tin for 10 minutes before turning them out onto a rack to cool completely.