Preheat oven to 325°F. Line muffin tins with liners (recipe makes 6 muffins) Measure the butter, then place in a medium sized microwave-safe bowl and heat until melted. Whisk in the egg and vanilla. Whisk in both the granulated sugar and light brown sugar until smooth.
Mash the bananas with a fork to remove large clumps. Measure out 1 cup of mashed banana and mix it in with the other liquid ingredients. Add the baking soda, salt and flour and whisk until just combined. If you’re using chocolate chips, stir them in now.
Divide the batter between the liners, filling each one almost full. Bake in preheated oven for 18-22 minutes, or until a toothpick inserted in the middle of a muffin comes out clean.