In a medium bowl, whisk the eggs, milk and salt. Working in batches, melt some of the butter in a large nonstick skillet over moderately-high heat. Dip the bread slices in the egg mixture and quickly transfer to the skillet. Cook 2 slices of the bread until golden brown, and then flip over and cook until golden brown on the other side. Spread 2 tablespoons of the chocolate hazelnut spread on 1 piece of the bread, and place the other piece of cooked bread on top to enclose the chocolate hazelnut spread. Gently press down with a spatula so the pieces stick together. Repeat with the remaining ingredients. Serve with grade B maple syrup or powered sugar.