Position a rack in the center of the oven and preheat to 350 degrees F. Butter and flour a 9-by-5-by-3-inch loaf pan.
Put the chocolate in a heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl on the pan (without touching the water). Stir occasionally until melted and smooth. Keep warm.
Whisk the flour, baking soda, baking powder, cinnamon, and cloves in a medium bowl. Stir the vanilla and half the bananas into the buttermilk.
Beat the butter and sugar in a large bowl, with an electric mixer on medium-high speed until light and fluffy, about 4 minutes. While mixing, drizzle in the oil and beat until incorporated. Beat in the egg.
While mixing slowly add the flour mixture to the butter in 3 additions, alternating with the buttermilk mixture in 2 parts, beginning and ending with the flour. (Scrape the sides of the bowl between additions, if needed). Fold in the remaining bananas. Gently swirl the melted chocolate into the batter so that it has a marbled effect. Scoop the batter into the prepared pan. Bake until a toothpick inserted in the center comes out clean, about 50 minutes. Cool in the pan on a rack before unmolding and slicing.