Prep the pan and oven. Heat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line it with a parchment sling for easy removal.

Warm the Nutella slightly. Microwave for 10–15 seconds or place the jar in warm water. You want it pourable but not hot.

Mash the bananas. In a large bowl, mash the bananas until mostly smooth with some small lumps. Measure to ensure about 1 1/2 cups.

Whisk the wet ingredients. Add granulated sugar, brown sugar, melted butter, eggs, sour cream, and vanilla. Whisk until smooth and glossy.

Combine the dry ingredients. In a separate bowl, whisk flour, baking soda, baking powder, and salt. This prevents clumps and ensures even rise.

Bring the batter together. Add the dry mixture to the wet and stir with a spatula just until no dry streaks remain. Do not overmix. Fold in any optional add-ins now.

Layer and swirl. Pour half the batter into the pan. Drizzle half the Nutella over the top in thin ribbons. Add the remaining batter, then the remaining Nutella. Use a butter knife to gently swirl in a figure-eight pattern. Stop before it blends entirely—you want distinct ribbons.

Bake. Bake for 55–70 minutes, until the top is set and a tester inserted in the center comes out with moist crumbs (avoid a Nutella pocket when testing). Tent with foil after 45 minutes if browning too quickly.

Cool properly. Let the loaf cool in the pan for 15 minutes, then lift out and cool on a rack for at least 1 hour before slicing. Slicing too soon makes it gummy.

Serve. Slice thickly and enjoy warm or at room temperature. A little salted butter or a sprinkle of flaky salt is a nice touch.