Whisk together the flour, granulated sugar, baking powder, baking soda and salt. Set aside.
Whisk together the melted and cooled butter, eggs, vanilla extract, milk, and mashed bananas.
Pour the dry ingredients into the wet ingredients and fold together until combined. Add the chocolate and fold to combine. Cover the bowl with plastic wrap and let the batter rest at room temperature for 30 minutes.
Fill the cups with the batter all the way to the top. Bake for 8 minutes at 425 degrees Fahrenheit, then drop the temperature to 350 degrees Fahrenheit and bake for another 8-10 minutes for regular sized muffins and 17-20 minutes for jumbo muffins.