Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
Mash the ripe bananas in a large bowl until smooth.
Add melted butter, sugar, eggs, and vanilla extract to the bananas; mix until combined.
In another bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually fold into the wet mixture until just combined.
Gently fold in dark chocolate chips.
Scoop the batter into prepared muffin tins, filling each about two-thirds full.
Bake for 18-20 minutes or until a toothpick inserted comes out clean.
