Heat the oven to 350℉/180℃, grease a 9 inch by 5 inch loaf pan, and dust the inside lightly with cocoa powder.
In a small bowl, whisk together the all-purpose flour, whole wheat flour, cocoa powder, baking powder, baking soda, and salt.
¾ cup (95 grams) all-purpose flour, ½ cup (62 grams) whole wheat flour, ½ cup (48 grams) natural unsweetened cocoa powder, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
In a separate large bowl or bowl of your mixer, beat together the melted butter with the granulated and brown sugars until smooth.
4 tablespoons (57 grams) unsalted butter, ½ cup (100 grams) granulated sugar, ½ cup (98 grams) brown sugar
Beat in the egg, vanilla, and mashed bananas, beating for a minute after each addition and scraping down your bowl as necessary.
1 large egg, 1 teaspoon vanilla, 2 cups very ripe banana
Stir the dry ingredients into the banana mixture just until no streaks of flour remain.
Fold in the chocolate chips.
1 cup (170 grams) chocolate chips
Poor the batter into the prepared loaf pan, and bake 50 to 60 minutes until a thin knife inserted into the center of the chocolate banana bread comes out with a few moist crumbs.
Let the bread cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.