Preheat the oven to 350℉. Line the cups of muffin tins with 18 muffin liners or grease with non-stick cooking spray.
In a large bowl using a handheld electric mixer, beat the butter and brown sugar on high speed until fluffy. Add the mashed bananas, eggs, and vanilla extract and beat until well combined.
Whisk together the flour, cocoa, baking soda, salt, and chocolate chips in a separate bowl before adding to the wet ingredients. Mix on low speed until the dry ingredients are just incorporated.
Scoop the batter into the prepared muffin tin, filling each cup to the top with the batter. Bake in preheated oven for 25-30 minutes or until a tester inserted into the top comes out without any raw batter. Let the muffins cool in the tin for 5 minutes before transferring to a wire rack to finish cooling. Enjoy!
Notes
You may use unsalted butter with ¼ teaspoon of additional salt.
In a pinch, you could use white sugar instead.
Chopped chocolate or chocolate chunks may be used instead of semisweet chocolate chips.