Make your filling by whisking your cornstarch and ½ cup of heavy cream in a small bowl. To prevent clumping ensure you are constantly whisking as you add the cornstarch.
Add your heavy cream, vanilla extract, Nutella, and salt into a medium saucepan. Cook over medium heat, whisking constantly until the mixture is well combined and smooth.
Add the cornstarch and cream mixture, whisking constantly to avoid lumps in the filling. Let the mixture thicken up for 2-3 minutes whisking constantly. Remove from your heat and continue whisking for 2 more minutes.