Fill up the juice with water so you have 2 cups/ 500ml of liquid. If you use frozen cherries use water instead. Remove 6 tablespoons of liquid and mix them with the cornstarch, sugar, and vanilla extract (affiliate link) to a thick paste.
Now heat up the cherries at a medium heat to boil. Stir in the cornstarch / cornflour paste and boil for 2 minutes. The cherry sauce should start to thicken.
Place the remaining juice in a small saucepan and bring to a boil. Once the liquid is boiling, remove the pot from the heat and stir in your cornstarch paste. Stir continuously with a whisk. Place back onto the heat and boil for another 1-2 minutes. You will see that the liquid will become paste-like.
Now add in the cherries, warm through and the sauce is ready to serve immediately.