Preheat the oven to 350°F (175°C).

Line the muffin tin with parchment paper or muffin liners.

In a mixing bowl, mash the ripe bananas with a fork until smooth.

Add melted butter, brown sugar, and granulated sugar to the mashed bananas. Stir until well combined.

Beat in the egg and vanilla extract until the mixture is smooth.

In another bowl, whisk together flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

If using, fold in chocolate chips.

Pour the batter into the prepared muffin tin, filling each cup about 2/3 full.

Bake for 18-20 minutes, or until a toothpick inserted comes out clean.

Let the muffins cool in the pan for a few minutes before transferring to a wire rack.