Here’s a banana bread that's soft, tender, and bursting with bananas, yet not overly sweet as so many versions are. The key is an intensely bitter cocoa-powder paste added to half of the batter, resulting in a deep, rich chocolate flavor that pairs perfectly with the sweetness of the bananas. The two batters are marbled together, producing a beautiful pattern and adding a sophisticated note to this all-American favorite. Because of the moisture that the bananas add, the loaf keeps well for at least 4 to 5 days at room temperature, and it freezes so well that you can put a couple of them away for unexpected visitors. Be sure the bananas are ripe and soft, with plenty of black spots speckling their skins, or you won’t have the burst of banana flavor you want. Whirl them in the food processor for an ultra-smooth purée, or mash thoroughly with a fork if you prefer.

Preheat the oven to 350°F and position an oven rack in the center. Lightly coat the loaf pan with melted butter or high-heat canola oil spray and line it with a piece of parchment paper that extends 1 inch beyond the long edge of both sides of the pan. Peel the bananas and place them in the bowl of the food processor. Process to a smooth purée. (Alternately, mash them in a bowl using a fork.) Measure out 1 cup of the purée and transfer it to a medium bowl, discarding the rest of the purée or saving it for another use. Add the buttermilk and vanilla and whisk just until blended. Set aside.

Use a fine-mesh strainer to sift the cake flour, baking soda, and baking powder together into a medium bowl. Whisk to blend well. Set aside. Place the cocoa powder in the small bowl. Pour the boiling water over the cocoa and stir until it forms a smooth paste-it should run thickly off the spoon. If it is too thick, add another tablespoon of boiling water and stir again. Set aside.

Place the butter and sugar in the bowl of the stand mixer. Beat on medium-high speed until the butter is very light, almost white in color, 4 to 5 minutes. Scrape down the bowl with a spatula. Turn the mixer to medium speed and add the eggs, 1 tablespoon at a time, completely blending in each addition before adding the next. About halfway through the eggs, turn off the mixer and scrape down the bowl with the spatula, then continue adding the rest of the eggs. Scrape down the bowl again.

With the mixer running on the lowest speed, add one-third of the flour mixture. Just as it is barely blended and you can still see a few patches of flour, add half the banana mixture. Repeat with the remaining flour and banana mixtures ending with the flour. Scrape down the bowl and finish blending the batter by hand.

Transfer half of the batter to the second medium bowl. Add the cocoa paste and, using the rubber spatula, gently but thoroughly blend it into the batter.

Drop alternating spoonfuls of dark and light batters into the prepared pan, then marbleize by using a spoon to gently turn the batter over in 3 places down the length of the pan.

Bake the banana bread for 55 to 65 minutes, until firm to the touch and a toothpick inserted into the center of the loaf comes out clean. Transfer to a rack to cool completely. When cool, remove from the pan, peel off the parchment paper, and cut slices by sawing gently with a serrated knife.