In a bowl mix chicken with cornstarch, rice wine vinegar and sesame oil. Set aside for 20 minutes in refrigerator. Mix flour, salt and pepper in another small bowl and set aside. Preheat oven to 475ºF. Prepare a baking sheet by lining with parchment paper.
After 20 minutes, stir chicken in marinade really well to absorb any liquid. Dredge the chicken pieces into the flour mixture and place them on the prepared baking sheet. Bake for 10-14 minutes, turning once. Remove from oven.
In a saucepan over medium-high heat add oil and garlic, stirring for 30 seconds. Add vinegar, chicken broth, sugar, soy sauce and sesame oil, bring to a simmer.
Gradually add cornstarch slurry while stirring constantly. Let simmer for 20 seconds until thick. Add baked chicken pieces and hoisin sauce tossing until fully coated with sauce. Serve garnished with green onions and sesame seeds.