Preheat your oven to 350°F. While it heats, gather all ingredients and prepare your workspace. Grease a 9x5 inch loaf pan or similar-sized baking vessel and set aside. Chop the hazelnuts into roughly 1/4-inch pieces and slice the 1/2 banana into 1/4-inch rounds; set both aside on a plate. Warm the Nutella slightly (about 20-30 seconds in the microwave or by sitting it in a warm spot) so it becomes pourable and easier to swirl into the batter.

Mash the 4 very ripe bananas in a large bowl until mostly smooth with just a few small chunks remaining—this texture creates better structure in the bread. Add the melted butter and stir until well combined. Mix in the brown sugar, sugar, and vanilla extract until the mixture is uniform and sweet-smelling. I like to melt my butter first before adding it to the bananas because it helps the sugars dissolve more evenly and creates a more cohesive batter.

In a separate bowl, whisk together the flour, baking powder, salt, and ground cinnamon. Whisking the dry ingredients ensures even distribution of the leavening agents and spices, which prevents dense pockets in the finished bread. Set this mixture aside.

Add the dry ingredient mixture from Step 3 to the wet banana mixture from Step 2, alternating with the milk: add about half the dry mixture, stir gently, then add the milk, then add the remaining dry mixture. Fold everything together until just combined—don't overmix, as this keeps the bread tender. A few small lumps of flour are fine and actually preferable to overworking the batter.

Pour about two-thirds of the batter into the greased loaf pan, spreading it evenly. Using a spoon or thin spatula, create a shallow channel down the center of the batter and spoon the warmed Nutella into it. Cover the Nutella with the remaining batter, smoothing the top gently. Now arrange the banana slices on top in a single layer or decorative pattern, and sprinkle the chopped hazelnuts over the bananas and any exposed batter areas. For extra flavor, I sometimes sprinkle a tiny pinch of sea salt over the hazelnuts to enhance their nuttiness and balance the sweetness.

Place the loaf pan in the preheated 350°F oven and bake for 1 hour and 10 minutes, until a toothpick inserted into the thickest part of the bread (avoiding the Nutella center) comes out clean or with just a few moist crumbs. The top should be golden brown and the bread should smell fragrant. If the top browns too quickly before the center is done, tent it loosely with foil for the final 15-20 minutes.

Remove the bread from the oven and let it cool in the pan for 15 minutes—this allows the structure to set so it won't fall apart when you turn it out. Then turn the bread out onto a wire rack and let it cool completely before slicing. Slicing while the bread is warm causes it to crumble, so patience here pays off with clean, beautiful slices.