Preheat oven to 350F. Line a 12-cup muffin tin with paper liners or spray with non-stick spray.

In the bowl of a stand mixer fitted with a paddle, cream together butter and brown sugar until fluffy. Add eggs one at a time, making sure each is incorporated before moving on; add vanilla and mashed bananas.

In a medium bowl, sift together flour, cocoa powder, baking soda, and salt.

Add the dry ingredients to the wet ingredients; mix until just combined.

Fold in chocolate chips with a rubber spatula.

Bake in preheated oven for 20 – 23 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Cool on a wire rack.