Preheat oven to 350ºF. Coat 12 muffin tins with cooking spray.
Beat together 2 tbsp light butter and 6 tbsp brown sugar. Beat in bananas, egg, vanilla, and milk.
In a separate bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
Gently fold in flour mixture to banana mixture, being sure not to over mix.
Divide batter among muffin pans. In a small bowl, combine remaining 1 tbsp light butter, 2 tbsp flour, 1 1/2 tbsp brown sugar, and walnuts. Mix until butter is evenly distributed and mixture is crumbly.
Divide crumble mixture evenly among tops of muffins.
Bake until a toothpick inserted into center comes out clean, about 22-26 minutes. Let cool in pans for 10 minutes removing from muffin tins to cool completely.