Apple Filling: By the time we've cooked the apple filling for so long, it really shouldn't have enough runny liquid that it needs to be thickened. However, if yours does for some reason - dissolve 1 teaspoon of corn starch in 1 teaspoon of water, and stir into the filling while it's still over medium heat. Cook, stirring for 45 seconds to 1 minute, until the juices are thickened up enough that you can spoon the filling into your cookies. Remove from heat immediately once thickened to your liking, and pour into a bowl to cool.