Whisk the sugar, soy sauce, vinegar, garlic, cornstarch, and pepper in a small saucepan over low heat. Bring to a rolling simmer, stirring frequently, and cook for 2-3 minutes. Remove from the heat then transfer to a small bowl and cool for 10-20 minutes.
Notes
This makes ¾ cup of sauce Be careful not to cook the teriyaki sauce for too long, otherwise, the sugars will crystallize and the sauce will become grainy.