1. Preheat oven to 350°.
2. 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.
3. 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.
4. 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.
5. Place bittersweet chocolate in a microwave-safe dish; microwave at high for 30 seconds. Drizzle bread with chocolate; let stand until set.