Make the crust: Preheat the oven to 350°F. Line an 8 by 8-inch metal baking pan with parchment, leaving an overhang. Pulse the crackers in a food processor until finely ground. Add the butter and pulse until moistened. Press the crumb mixture into the bottom of the prepared pan using the bottom of a measuring cup or glass. Bake for about 10 minutes. Place on a cooling rack to cool completely.
Make the filling: Reduce oven temperature to 325°F. In a large bowl, use an electric mixer fitted with the paddle attachment to beat the cream cheese, sugar, and Nutella until smooth and creamy, scraping down the sides and bottom of the bowl often, for at least 1 minute. Add the sour cream and cocoa powder and beat until well combined. Add the eggs one at a time, blending after each addition until combined. Add in Frangelico (or vanilla) and salt, and blend until just combined. Spread batter evenly over cooled crust. Bake at 325°F until just set at the edges but still slightly wobbly and jiggly in the center, about 35 minutes.
Make the topping: In a small saucepan set over medium heat, bring the cream to a simmer. Remove from heat and stir in the Nutella until completely smooth. Let cool and thicken before spreading over the cold cheesecake bars evenly. Return to fridge to firm up for at least 30 minutes.