Preheat the oven to 350°F. Mix together melted butter and mashed bananas. Mix in the sugars, egg and vanilla. Mix in the baking soda and salt, then add in the flour. Stir in the chocolate chips. Pour mixture into a 12-well muffin pan lined with paper liners. Bake for 20-23 minutes, or until toothpick inserted comes out clean (or more likely, coated in deliciously melted chocolate).