Line a 12 tin muffin liner with 12 liners or 2 liners with 6 muffin tins in each (Your preference) and set aside. Melt the butter and prepare to measure the rest of your ingredients as it cools.
In a large bowl, whisk melted and cooled butter, brown sugar, bananas, egg, egg yolk, and vanilla. Whisk until combined. It may be lumpy because of the bananas and that is okay!
1 egg yolk, 1 1/2 cups mashed bananas, 3/4 cup brown sugar, 1/2 cup melted butter, 1 tbsp vanilla, 1 egg
Add in flour, cinnamon, nutmeg, salt, cornstarch, baking soda, and baking powder. Using a spatula, gently fold until just combined. It's important not to over mix with muffins.
2 cups flour, 1 3/4 tsp baking powder, 1/2 tsp baking soda, 2 tsp cornstarch, 1/8 tsp ground nutmeg, 3/4 tsp cinnamon, 1/4 tsp salt
Toss in the mini chocolate chips and fold a few more times.
3/4 cups mini chocolate chips
Evenly divide the batter between 12 muffin liners. You can bake them all in 1 tin, or 6 in 2 tins for a taller rise.
Sprinkle extra mini chocolate chips and sugar on it.
Bake the muffins at 400℉ for 5 minutes, and then turn the oven down to 350℉ and continue baking for 12-15 minutes.
Allow the muffins to cool, and enjoy! These muffins stay good on your counter for up to 5 days and in the refridgerator in an airtight container for up to 2 weeks.