Preheat oven to 350°F. Butter a 9×5-inch loaf pan.
Chop the walnuts.
Mash the bananas in the bottom of a large bowl, enough for 1 cup. Melt the butter, then whisk it into the bananas along with the brown sugar, egg, and vanilla extract.
Add the baking soda, kosher salt, all-purpose flour, and Dutch-process cocoa powder to the bowl and stir together with a spoon.
Pour the batter into the loaf pan and top with chopped walnuts.
Bake 55 to 65 minutes (insert a toothpick into the center of the loaf: if it comes out clean, it’s done!) Cool in the pan for 15 minutes. Then run a knife around the edge and invert the loaf onto a cooling rack. Serve warm or at room temperature. The bread keeps up to 4 days at room temperature wrapped in aluminum foil.