In a large microwave-safe bowl, combine the finely chopped chocolate, hazelnut spread, heavy cream, hazelnuts, butter, and salt. Microwave in 30-second intervals, stirring the mixture in between with a non-stick spatula. (If you have a powerful microwave, microwave in 15-second intervals so the chocolate does not burn.) Once the mixture is fully melted, pour over the top and sides of the loaf, using an offset spatula to patch up any spots you missed. When the entire loaf is covered, place the sheet pan or cooling rack with the loaf in the refrigerator to set for at least 10 minutes.