Preheat the oven to 350 degrees F. Lightly grease a 9x9 inch pan and line with parchment paper, allowing 2 sides to hang over the edges.
Prepare the crust: in a medium bowl, whisk together the sugar, flour, and salt. Use clean hands, a fork or a pastry blender to mix in the butter until its well blended with the other ingredients. Press the mixture firmly and evenly into the bottom of the prepared pan. Bake for 15 minutes or until the edges are lightly golden brown.
While the crust is cooking, prepare the filling: In a stand mixer fitted with the paddle attachment, combine the softened cream cheese, vanilla, sugar, egg and 1 tablespoon Nutella, scraping down the sides to ensure that the ingredients are evenly combined. Continue mixing on medium speed until well blended, 1-2 minutes.
Place the remaining 3 tablespoons of Nutella in a microwave-safe dish and heat in 15 second intervals, stirring each time, until the spread has thinned out and will be easier to swirl onto the top of the cheesecake mixture.
Remove the pan from the oven and pour the filling over the crust, using a spatula to spread evenly. Drizzle the warm Nutella on top and use the tip of a spoon to swirl decoratively, taking care not to disturb the crust.
Bake for an additional 15-20 minutes or until the filling is set. Cool for 30 minutes and then refrigerate for 30 minutes. Slice into 16 bars and serve. Store in the refrigerator.