Whisk hot sauce, brown sugar, vinegar, soy sauce, water, and red pepper flakes in a bowl; set aside.
Pat chicken dry. Toss with salt, pepper, garlic powder, and cornstarch until evenly coated.
Heat oil in a large skillet over medium-high. Sear chicken in two batches until browned and cooked through, 6–8 minutes total.
Reduce heat to medium. Pour in the sauce and simmer, stirring, until glossy and slightly thick, 2–3 minutes.
Return chicken to the skillet and toss to coat. Cook 1–2 minutes until sticky.
Finish with sesame seeds and green onions. Serve hot over rice, if desired.