Cut chicken thighs into 1–1½ inch chunks. Toss with salt and cornstarch.
Heat oil in a skillet. Lightly brown chicken for 2–5 minutes. Remove from pan.
In the same skillet, sauté garlic, ginger, and pepper flakes for 1 minute.
Add stock, soy sauce, brown sugar, and vinegar. Simmer 6–8 minutes until reduced.
Return chicken to skillet. Cook 5 minutes, stirring, until sauce coats chicken.
Toss with sesame seeds and garnish with scallions.
