Preheat the oven to 350°F (175°C). Line a muffin tin with paper liners.
In a large bowl, mash the bananas and mix in granulated sugar, brown sugar, melted butter, eggs, and vanilla extract.
In another bowl, whisk together flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips.
Divide the batter evenly into the muffin liners, filling each about 3/4 full.
Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean.
Let the muffins cool for 5 minutes in the pan before transferring to a wire rack to cool completely.