Mash ripe bananas with a fork in a large bowl. Add melted butter, eggs, honey, milk, brown sugar, vanilla, baking powder, and salt. Mix well. Stir in protein powder until well combined. Stir in flour ½ cup at a time. Once everything is well mixed, spoon into muffin tins, dividing evenly. Top each muffin with a pinch of chopped walnuts.