Use a light hand when swirling the batter; too much swirling may lose the stripe effect.
5 tablespoons chocolate-hazelnut spread (such as Nutella)
3 tablespoons plus 1 teaspoon canola oil, divided
3 tablespoons butter, softened
1/2 cup packed brown sugar
2 medium ripe bananas, sliced
2 large eggs
6.75 ounces all-purpose flour (about 1 1/2 cups)
3/4 teaspoon baking soda
1/2 teaspoon baking powder
1/2 teaspoon salt
2/3 cup whole buttermilk
Baking spray with flour (such as Baker's Joy)
1/4 cup hazelnuts, coarsely chopped
1 ounce bittersweet chocolate, finely chopped
How to Make It
Preheat oven to 350°.
Combine chocolate-hazelnut spread and 1 teaspoon oil in a microwave-safe dish; microwave at HIGH for 30 seconds or until melted. Stir. Combine 3 tablespoons oil, butter, brown sugar, and banana in a large bowl; beat with a mixer at medium-high speed until well blended. Add eggs, 1 at a time, beating well after each addition.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking soda, baking powder, and salt. Beating at low speed, add flour mixture and buttermilk alternately to banana mixture, beginning and ending with flour mixture; beat just until combined. Scrape half of batter into a 9 x 5-inch metal loaf pan coated with baking spray, and top with chocolate-hazelnut spread mixture. Spread remaining batter over chocolate mixture. Using a wooden pick, swirl batter. Sprinkle batter with hazelnuts.
Bake at 350° for 55 minutes or until a wooden pick comes out with moist crumbs clinging. Cool 10 minutes in pan on a wire rack. Remove bread; cool on wire rack.
Place bittersweet chocolate in a microwave-safe dish; microwave at high for 30 seconds. Drizzle bread with chocolate; let stand until set.