Start by preheating your oven to 180°C (350°F) and lining a muffin tin with paper liners.
Use a fork or potato masher to mash the bananas in a large bowl until mostly smooth, leaving a few small lumps for texture and aroma.
Chop the dark chocolate into small chunks or use chocolate chips, then set aside for folding in later.
In a separate bowl, whisk together the flour, cocoa powder, brown sugar, baking powder, and salt until evenly combined.
Add the egg, melted butter, and vanilla extract to the mashed bananas, then stir until well combined and slightly creamy.
Pour the wet mixture into the bowl with the dry ingredients, then gently fold everything together until just combined—don’t overmix to keep the muffins tender.
Gently fold in the chopped dark chocolate, distributing it evenly throughout the batter. If it's too thick, add a splash of milk to loosen it slightly.
Spoon the batter into the prepared muffin cups, filling each about three-quarters full to give space for rising.
Bake in the preheated oven for 20-25 minutes, until the muffins have risen, are golden on top, and a toothpick inserted into the center comes out with moist crumbs.
Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes, then transfer them to a wire rack to cool slightly.
Serve the muffins warm or at room temperature, enjoying the gooey pockets of chocolate and the tender, fragrant crumb.
