Preheat the oven to 375F. Lightly grease the wells of 2 muffin pans.
In a medium mixing bowl, combine the melted butter, peanut butter, and mashed banana. Whisk well to combine. Add the brown sugar and continue to stir until well incorporated.
Add the eggs, vanilla, and milk and whisk to combine. Next add the salt, baking soda, and flour and mix with a spatula until just incorporated, being sure not to over mix.
If you’re using chocolate chips or other mix-ins, fold them in now.
Scoop the batter into the muffin pans and bake for 18-20 minutes or until a toothpick inserted into the middle comes out clean. Let the muffins cool for 5 minutes in the pan before removing and serving.