Preheat the oven to 350°F. Grease a 9x5-inch loaf pan or line it with parchment paper.
In a large bowl, whisk melted butter and sugar until combined. Beat in eggs and vanilla extract.
Mix in cocoa powder and mashed bananas until smooth.
Stir in flour, baking soda, and salt until just combined.
Fold in both types of chocolate chips.
Pour the batter into the prepared loaf pan and spread evenly.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.