In a small saucepan, whisk together the cornstarch and cold water until smooth and no lumps. Then, whisk in the sugar, soy sauce, vinegar, garlic, and ground black pepper.
Bring to a boil over medium heat then reduce it and simmer until thickens. You will end up with about 1 cup of sauce.
To freeze it, wait until room temperature (or cooled in frig), then place in storage container. I usually make a batch, use one immediately and freeze the rest in 2 different portions.