Preheat oven to 425℉ and line 12 muffin cups with paper liners.
In a large bowl, mix together the mashed bananas and butter. Add the brown sugar, white sugar, egg, and vanilla and mix until combined.
In a separate bowl (in full disclosure I just do this in the same bowl), whisk together the flour, baking powder, baking soda, and salt. Add to the wet ingredients and mix until just combined. Stir in the sour cream and chocolate chips.
Scoop into the muffin liners with the muffin batter almost all the way to the top. Add a few more chocolate chips on top.
Bake for 5 minutes, then reduce oven temperature to 350℉ and bake for 11-13 minutes or until centers are set and muffins are a light golden brown. Let muffins cool in the pan. Store leftovers in an airtight container.