Add water, soy sauce, balsamic vinegar, sriracha sauce (or red pepper flakes), brown sugar, sesame oil, and garlic to a medium sauce pan. Cover and simmer for about 20 minutes until reduced down to half.
Uncover sauce pan and bring heat to medium-high, stirring regularly until thickened (about 8-10 minutes) then remove from heat. *Be careful not to let the sauce burn when it thickens.
Grease and preheat the grill. Season both sides of chicken with salt and pepper to taste. Grill for 5-6 minutes on each side until cooked through.
Drizzle chicken (or toss) with the sauce and garnish with cilantro, sesame seeds, or green onions if desired. Serve immediately.