3 medium-to-large very ripe bananas
1/2 cup (115 grams) butter, melted
3/4 cup (145 grams) brown sugar (I use dark here; either light or dark work)
1 large egg
1 teaspoon pure vanilla extract
1 teaspoon baking soda
1/4 teaspoon table salt
1/2 teaspoon ground cinnamon (optional; I skip it)
1 cup (125 grams) all-purpose flour
1/2 cup Dutch-process cocoa powder
1 cup (about 6 ounces or 170 grams) semisweet or bittersweet chocolate chunks or chips
Heat your oven to 350°F. Butter a 9×5-inch loaf pan, or spray it with a nonstick baking spray.
Mash bananas in the bottom of a large bowl. (You’ll have a little over 1 cup mashed banana total.) Whisk in melted butter, then brown sugar, egg, and vanilla. Place baking soda, salt, cinnamon (if using), flour and cocoa powder in a sifter or fine-mesh strainer and sift over wet ingredients. (My cocoa is almost always lumpy, so this is essential for me.) Stir dry and wet ingredients with a spoon until just combined. Stir in chocolate chunks or chips.
Pour into prepared pan and bake 55 to 65 minutes, until a tester or toothpick inserted into the center of the cake comes out batter-free. (A melted chocolate chip smear is expected, however.) Cool in pan for 10 to 15 minutes, then run a knife around the edge and invert it out onto a cooling rack. Serve warm or at room temperature.
The banana bread will keep for up to 4 days at room temperature. I keep mine wrapped in foil. It goes without saying that it has never actually lasted that long.