Combine all icing ingredients except sugar in a small sauce pan over medium heat. Once heated, slowly stir in sugar until dissolved.
Remove from heat and place in fridge/freezer to cool and thicken until use
Notes
Note: If you make up the apple filling and let sit, it will produce a lot of liquid -- I sometimes use this liquid + powdered sugar to make the icing and skip the ingredients here. The amount of powdered sugar will depend on how juicy your apples are, but you can add milk if you need more.