In a medium bowl, whisk together the cornstarch and cold water until the cornstarch is fully dissolved. Add the brown sugar, soy sauce (or tamari), honey, ground ginger, and garlic powder, and stir until well combined.
Pour the mixture into a small saucepan and bring it to a simmer over medium-high heat. Once simmering, reduce the heat to medium and continue cooking for about 5 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon. Note that the sauce will continue to thicken as it cools.
