Add all sugar, water, and raspberries to a small saucepan. Bring to a boil, stir constantly and mash the berries with the back of your spoon.
Sustain a low-medium boil for 10 minutes. The mixture will turn dark red and thick.
Blitz with an immersion blender. Allow the mixture to cool a little.
Stir in the cornstarch and whisk briskly to dissolve completely. Return to heat and bring the sauce back to a boil, stirring constantly. It will thicken when it boils, cook another minute or so.
If desired, strain through a wire mesh strainer to remove raspberry seeds. Recipe yields about 1 cup sauce.
Store covered in the refrigerator.
