Preheat oven to 425° degrees. Line a muffin tin with 12 muffin liners and set aside.
Melt the butter in a medium size microwaveable bowl and set aside to cool for 2 minutes.
Un-peel and mash the bananas into the melted butter using a fork. If the bananas are not overripe, mash them in a separate bowl or cutting board.
Add the eggs and vanilla and whisk to combine.
In another bowl combine the flour, brown sugar, baking soda, baking powder, cocoa powder, and salt and mix lightly. Add the wet ingredients into the dry ingredients and mix with a spatula until just combined. Do not over-mix or the muffins will be tough.
Take the last 2 teaspoons of flour and dust the chocolate chips with them- this will ensure they don’t sink to the bottom of the muffins! Pour the chocolate chips into the muffin batter and lightly mix. Using a regular sized ice cream scoop or a ¼ cup measuring cup, evenly fill the muffin tin 3/4 way full. Do not over fill!
Bake at 425° degrees for 8 minutes and then lower the oven temperature to 350°F (dont open the oven!) and continue baking for 10 minutes. The muffins are done when the edges are crisp and the tops are firm.