Preheat oven to 375°F. Line a 12-cup muffin pan with liners.
Whisk flour, cocoa, baking soda, baking powder, salt, and espresso powder in a bowl.
Mash bananas in a large bowl until mostly smooth.
Whisk in granulated sugar, brown sugar, melted butter, yogurt, eggs, and vanilla until combined.
Fold the dry ingredients into the banana mixture just until no dry streaks remain.
Stir in dark chocolate, keeping the batter thick and lumpy.
Divide batter among liners, filling each about 3/4 full. Sprinkle with coarse sugar if using.
Bake 16–18 minutes, until a toothpick comes out with a few moist crumbs (melted chocolate is fine).
Cool in the pan 5 minutes, then transfer muffins to a rack to finish cooling.