Preheat the oven to 400°F and either grease the bottoms of 12 standard-sized muffin cups or line them with papers. Set aside.
Prepare the streusel topping by combining the flour, ground walnuts, brown sugar and cinnamon in a small bowl. Use a pastry blender or two forks to cut in the butter until the mixture is crumbly. Set aside.
Whisk the flour, brown sugar, ground walnuts, baking powder and salt together until well combined.
In a separate, larger bowl whisk the milk, vegetable oil, egg and vanilla together until thoroughly blended. Add the flour mixture all at once and stir to combine. Do not overmix, it's okay for the batter to be lumpy.
Fold in the mashed banana and once again, don't overmix. Divide the batter evenly between the prepared muffin cups and top each with a portion of the streusel topping.
Bake for 18 to 22 minutes, until golden brown and a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes, then transfer to a cooling rack.