Cornstarch can thicken any sauce without altering flavor, including caramel sauce. Overcooking the sauce after adding the cornstarch will cause it to thin again, so add the cornstarch at the end of the cooking time. Recipes for caramel sauce call for an addition of cream at the end. This cream partially thickens the sauce, and by mixing cornstarch with it, the sauce will become even thicker. The standing time for the caramel sauce is important, since it gives the cornstarch a chance to swell and make the sauce thicker.
Whisk together the cornstarch and the cold, heavy cream or milk from the caramel sauce recipe.
Prepare the caramel sauce according to your recipe up to adding the cream.
Stir the cream and cornstarch mixture into the warm caramel.
Cook the caramel sauce over medium heat, stirring continuously for one minute.
Remove the sauce from the heat for five minutes to cool and thicken.