In a small bowl, stir together sugar and cornstarch. This helps to keep the corn starch fro clumping in the cherry juice.
Add maraschino cherries to a medium saucepot and bring to a gentle simmer over medium heat.
Sprinkle the sugar and corn starch mixture over the cherries and stir to fully incorporate.
Crush cherries slightly with a fork or a potato masher.
Cook over medium heat until thickened. Allow to fully cool.
This is easy to make ahead and store in the fridge. It will become very thick once cold, and if you find it's gotten too thick to add to your cake, stir in some reserved maraschino cherry juice to thin.