Microwave the Nutella and 1 teaspoon of oil until melted, about 30 seconds.
In the bowl of the stand mixer, beat the 3 tablespoons of oil, butter, brown sugar, and bananas until well-blended. Add 1 egg at a time, blending after each.
Whisk the flour, baking soda, baking powder, and salt in the medium bowl. On slow speed, add ⅓ of the flour, half the buttermilk, then ⅓ of the flour, half the buttermilk, and the remaining flour. Spread half of the batter in the prepared pan, then top with the Nutella mixture. Spread the remaining batter over the Nutella. Use a toothpick to gently swirl the batter, but not too much to keep the Nutella stripe effect. Sprinkle the chopped hazelnuts on top.
Bake in the preheated oven until a toothpick comes out with a few moist crumbs clinging, about 55 minutes. Cool bread in the pan on a wire rack for 10 minutes, then run a plastic knife around the edges to release. Turn the bread out onto the wire rack. Warm the chopped chocolate and drizzle over the bread (optional). Let the bread rest until cooled.