Heat oven to 350°F. Butter a 9×5-inch loaf pan, or coat it with a nonstick baking spray.
Melt butter in the bottom of a large bowl in the microwave. Mash the bananas right into it until mostly smooth. Whisk in brown sugar, egg, vanilla, baking soda, and salt until thoroughly combined. Add 1 cup of the flour, stirring just until it disappears.
Pour half of batter into a second bowl. (You can eyeball it, it’s fine. Or you can know that my batter halves were roughly 365 grams each, but weights will vary with banana sizes.) Into one bowl, stir the remaining 1/4 cup of flour and ground cinnamon. Into the other bowl, stir in the cocoa powder and chocolate chips.
Dollop batters in large alternating spoonfuls into bottom of prepared loaf pan. Attempt to “checkerboard” the rest in, roughly meaning that you’ll drop a chocolate batter dollop on top of a chocolate-free one and vice-versa until both batters are used up. Use a butter knife or small offset spatula to make a few figure-8s through the batters, marbling them together — but just a little, say, 2 to 3 figure-8s. Any more and the swirls may not look distinct when you cut the cake.
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 minutes, then run a knife around the edge and invert it out onto a cooling rack. Serve warm or at room temperature.
Do ahead: The banana bread will keep for up to 4 days at room temperature. I keep mine wrapped in foil.