To prepare blueberry sauce, mix sugar, cornstarch, blueberries and water in a small bowl and bring to a boil. Reduce heat and simmer until mixture begins to thicken, about 2 minutes. Remove from heat. Let cool completely.
Toast waffles if they are leftovers. Let cool slightly.
Scoop ice cream on a waffle. Top it with a spoonful of blueberry sauce, followed by another waffle.