Preheat the oven to 350F and grease a mini muffin pan with a light layer of butter or oil.
Mash the bananas and measure. Pour into a medium mixing bowl and add the cooled, melted butter. Mix well with a hand mixer until smooth.
Add the sugar, brown sugar, egg, and vanilla to the mashed banana and melted butter. Mix well until no lumps remain.
Add in the baking soda and salt. Fold in with a silicone spatula.
Add the flour and chocolate chips at once, folding in gently with the silicone spatula.
Use a #40 scoop (2 tbsp. size) to portion the batter into mini muffin tins. If you do not have a scoop, fill the mini muffin tins until they are full and slightly mounded.
Bake in the oven for 10-14 minutes, or until a toothpick comes out clean (well, cleanish. You will still see the melted chocolate chips).
Cool the mini muffins in the pan for about 10 minutes before removing gently with a butter knife and cooling for an additional 30 minutes or so on a cooling rack.
Store in the fridge, covered, for about a week. You can freeze if you want to keep these on hand as a quick snack or lunch box option.