In a large bowl, coat the chicken pieces with cornstarch, ensuring each piece is well-covered.
Heat vegetable oil in a pan over medium-high heat. Add the coated chicken pieces and cook until golden brown and crispy, about 4-5 minutes per side. Remove the chicken from the pan and set aside, on top of a few paper towels. Remove any excess oil from the skillet.
In the same pan, combine Thai sweet chili sauce, soy sauce, rice vinegar, honey, minced garlic, grated ginger, and red pepper flakes. Stir well and let it simmer for 2-3 minutes until the sauce thickens.
Add the cooked chicken back into the pan, tossing it in the sauce until each piece is coated evenly. Cook for an additional 2-3 minutes. Serve the Thai Sweet Chili Chicken garnished with sliced green onions and sesame seeds.