Stir the sugar and cinnamon together in a small bowl; set aside for dusting.
Whisk cornstarch and water to make a thin slurry.
Lay out a few wonton wrappers. Add a small spoonful of chocolate-hazelnut spread to the center of each.
Brush wrapper edges lightly with the slurry. Fold into a triangle, press out any air, and seal firmly. Crimp edges for extra security.
Repeat with remaining wrappers. Chill the filled dumplings for 10 minutes while you heat the oil.
Heat oil in a deep, heavy pot to 350°F over medium heat.
Fry dumplings in batches, 45–60 seconds per side, until deep golden and crisp. Avoid overcrowding.
Transfer to a rack or paper towels. Immediately dust with the cinnamon sugar.
Serve warm, sprinkled with chopped hazelnuts if using.