Stir brown sugar, Tamari, rice wine vinegar, Gochujang paste, sesame oil, ginger, garlic, honey and black pepper together in a saucepan. Bring to a boil over medium-high heat, stirring constantly.
Whisk cornstarch and water together in a small bowl until the cornstarch dissolves. Pour the cornstarch slurry into the boiling Tamari mixture. Continue to stir the sauce.
Reduce the heat to low and cook until the sauce thickens, about 3 - 4 minutes.
Use immediately or store covered in the refrigerator for up to 2 weeks.
