Prep the pan and oven: Heat oven to 350°F (175°C).
Line a 9x5-inch loaf pan with parchment and lightly grease the sides.
Mash the bananas: In a large bowl, mash bananas until mostly smooth with a few small lumps for texture.
Whisk wet ingredients: Add eggs, oil, brown sugar, granulated sugar, and vanilla. Whisk until well combined and glossy.
Combine dry ingredients: In the same bowl, sprinkle in flour, cocoa powder, baking soda, baking powder, salt, and espresso powder. Stir gently with a spatula just until no dry streaks remain.
Fold in chocolate (and nuts): Stir in dark chocolate chips and nuts, if using.
Do not overmix.
Fill the pan: Scrape batter into the prepared loaf pan. Smooth the top. Sprinkle a few extra chocolate chips on top if you like.
Bake: Bake for 55–65 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (melted chocolate on the stick is okay).
Cool: Let the bread cool in the pan for 15 minutes, then lift out using the parchment and cool on a rack for at least 45 minutes before slicing.
Slice and serve: Use a serrated knife for clean slices.
Serve slightly warm for melty chocolate.