Preheat the oven to 350-degrees and line a muffin tin with papers. Melt the butter and set aside to cool.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt. Add in the butter, mashed bananas, egg, and milk, taking care not to over mix. Fold a ½ cup of the walnuts, reserving the rest for the top of the muffins.
Using a muffin scoop or ¼ measuring cup, fill the muffin cups about ⅔ of the way full. Sprinkle the tops of the muffin batter with the remaining ¼ cup of walnuts.
Bake for 28-32 minutes until a tester comes out clean.