1. Preheat oven to 350°F. Line an 8 by 8-inch metal baking pan with parchment paper, leaving an overhang.
2. In a food processor, pulse the chocolate graham crackers until finely ground. Add the melted butter and pulse until moistened.
3. Press the crumb mixture into the bottom of the prepared pan. Bake until fragrant, about 10 minutes. Cool completely.
4. Reduce oven temperature to 325°F.
5. In a large bowl, use an electric mixer fitted with the paddle attachment to beat the cream cheese, sugar, and Nutella on medium-high speed until smooth and creamy.
6. Add the sour cream and sifted cocoa powder, and beat until well combined. Add the eggs one at a time, blending after each addition. Scrape down the sides and bottom of the bowl.
7. Add the Frangelico (or vanilla) and salt, and blend until just combined.
8. Spread the batter evenly over the cooled crust. Tap the pan against the counter to release air bubbles.
9. Bake at 325°F until just set at the edges but still slightly wobbly in the center, about 35 minutes. Turn off the oven and crack open the door for 10 minutes. Cool completely on a wire rack.
10. Cover with foil and refrigerate for at least 6 hours or overnight.
11. In a small saucepan set over medium heat, bring the heavy cream to a simmer. Remove from heat and stir in the Nutella until smooth. Let cool and thicken before spreading over the cold cheesecake bars.
12. Return to the fridge to firm up for at least 30 minutes. Cut into squares.
Enjoy these Nutella Cheesecake Bars!