Prepare the Pan: Preheat the oven to 350°F. Line a 9×5-inch loaf pan with parchment paper, then set it aside.
Combine Dry Ingredients: In a small bowl, whisk together the flour, baking powder, and salt until evenly combined.
Cream Butter Sugar: In a large bowl, beat the room-temperature butter, granulated sugar, and brown sugar with a handheld mixer on medium-high speed for 2 minutes, until the mixture is smooth and creamy. Scrape down the sides and bottom of the bowl with a rubber spatula. Add Eggs Vanilla: Add the room-temperature eggs and beat on high speed for 1 minute. Mix in the vanilla extract until incorporated.
Build the Batter: Add half of the dry mixture to the wet ingredients and mix until barely combined. Pour in the buttermilk and mix, then add the remaining dry ingredients and mix just until combined. Avoid overmixing.
Fold in Hazelnuts: Gently fold the chopped hazelnuts into the batter if using.
Layer the Batter: Transfer half of the batter to the prepared loaf pan and spread it into an even layer.
Soften Nutella: Warm the Nutella in the microwave for 15–20 seconds, just until it becomes slightly softened and easier to drizzle.
Create the Swirl: Drizzle half of the softened Nutella over the batter. Add the remaining batter to the pan, then drizzle the rest of the Nutella over the top. Use a knife to gently swirl the Nutella through the batter. Sprinkle additional hazelnuts over the top if desired. Bake the Loaf: Bake at 350°F for 55–75 minutes, or until a skewer or knife inserted into the center comes out clean. If the top begins browning too quickly, loosely tent the loaf with aluminum foil.
Cool Completely: Place the loaf pan on a cooling rack and allow the bread to cool for at least 1 hour. Once cooled, remove the bread from the pan and serve.
