Preheat the oven to 350℉. Line muffin tins with 24 paper liners.
In a large bowl, cream together the butter and sugars until light and fluffy. Mix in the egg, sour cream, banana and vanilla until fully combined.
In a separate bowl, mix together the flour, cinnamon, salt, baking powder, baking soda and cocoa powder.
Slowly mix the dry ingredients into the wet ingredients until just combined.
Fold in around ½ cup of chocolate chips and ½ cup white chocolate chips.
Divide the batter evenly in the prepared muffin cups. Sprinkle the remaining chocolate chips and white chocolate chips on top.
Bake in the preheated oven for 20-22 minutes, or until a toothpick inserted into the center of a muffin comes out with a few crumbs on it.
Allow to cool in the pan for around 2 minutes before transferring to wire racks to cool completely.
Notes
To store, place the muffins in an airtight container with a paper towel at room temperature for up to 3 days, or in the fridge for up to 1 week.
To freeze, place the cooled muffins in a freezer-safe bag. Try to remove as much air as possible from the bag, then place in the freezer for up to 3 months. When ready to eat, thaw at room temperature or microwave for around 15 seconds until warm.
Use brown bananas. The browner and spottier they are, the sweeter and more moist your muffins will turn out.
Do not overmix. This will result in a very dense and tough muffin. Make sure to mix the ingredients until just combined.
Use room temperature ingredients to keep the batter smooth and prevent the butter from re-solidifying.
Use a large cookie scoop to scoop the batter into the muffin pan. I like this cookie scoop to cleanly divide the muffin batter into the muffin pan.