Add apple cider vinegar, brown sugar, ketchup, soy sauce, cold water (or pineapple juice ), garlic, and cornstarch to a small saucepan.
Give it all a good stir until the cornstarch dissolves.
Turn on the heat to medium and stir frequently until it starts to boil. Allow it to boil for about 30 seconds. (At this point the sauce will thicken and be shiny.) Remove from heat and use as a sauce or dip.
Video
Notes
When made with 1 tbsp of cornstarch, this sauce will be thick enough to stick to your food, but not so thick that it's gloppy when cool. If you want a thick sauce use 2 tbsp of cornstarch. If you want a thinner sauce use 1/2 tbsp of cornstarch.