Preheat the oven to 350°F. Place muffin liners in a muffin tin.
Mash the bananas in the bottom of a large bowl, enough for 1 cup. Melt the butter, then whisk it into the bananas along with the brown sugar, egg, and vanilla extract.
In a separate bowl, mix the all-purpose flour, Dutch-process cocoa powder, baking soda, and kosher salt. Add to the bowl with the bananas and stir together with a spoon until a sticky dough forms. Pour the batter into the prepared muffin tin. Sprinkle the tops with turbinado sugar (or lightly with brown sugar).
Bake for 25 to 30 minutes, until a toothpick comes out clean. Allow to cool for a few minutes in the muffin tin, then move to a cooling rack to cool to room temperature before serving.