Step 1: Prepare the Crust

1. Preheat your oven to 325°F (160°C). Grease a 9×9-inch baking pan and line it with parchment paper, leaving overhangs on two sides for easy removal.
2. In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix until the crumbs are evenly moistened.
3. Press the mixture firmly into the bottom of the prepared pan, creating an even layer. Use the back of a measuring cup for a smooth surface.
4. Bake the crust for 10 minutes, then set aside to cool slightly.

Step 2: Make the Cheesecake Filling

1. In a large mixing bowl, beat the softened cream cheese and sugar with an electric mixer on medium speed until smooth and creamy.
2. Add the vanilla extract and eggs, one at a time, mixing on low speed until just combined.
3. Reserve 1/2 cup of the cheesecake mixture and set aside. Add the Nutella to the remaining mixture and beat until fully incorporated.

Step 3: Assemble the Bars

1. Pour the Nutella cheesecake filling over the prepared crust, spreading it evenly with a spatula.
2. Dollop the reserved plain cheesecake mixture on top, then use a toothpick or skewer to create swirls for a marbled effect.

Step 4: Bake the Cheesecake Bars

1. Bake in the preheated oven for 30–35 minutes, or until the edges are set and the center has a slight jiggle.
2. Turn off the oven and leave the bars inside with the door slightly ajar for 1 hour to cool gradually.

Step 5: Chill and Slice

1. Transfer the pan to a wire rack and let it cool to room temperature. Then refrigerate for at least 4 hours, or overnight, to set completely.
2. Use the parchment paper overhangs to lift the bars out of the pan. Slice into 16 squares with a sharp knife, wiping the blade clean between cuts for neat edges.

Step 6: Add the Toppings

1. Drizzle melted Nutella over the top of the bars and sprinkle with chopped hazelnuts for added texture and flavor.
2. Serve with whipped cream if desired.