Prepare the Chicken. Cut the chicken breasts or thighs into bite-sized pieces. Pat them dry with paper towels to help the coating stick better.
Coat the Chicken. In a shallow bowl, beat the egg. In another bowl, add the cornstarch. Dip each chicken piece into the beaten egg, then dredge thoroughly in the cornstarch, shaking off any excess. This coating will ensure a crispy finish.
Fry the Chicken. Heat the vegetable oil in a large skillet over medium-high heat. Fry the chicken pieces in batches until golden brown and crispy, about 5-7 minutes per batch. Remove and drain on paper towels.
Make the Sauce. Heat a tablespoon of oil in a skillet. Add the minced garlic and ginger, sautéing for 1-2 minutes until fragrant. Stir in the sweet chili sauce, soy sauce, rice vinegar, and brown sugar. If you prefer extra heat, add red pepper flakes. Bring to a simmer.
Combine Chicken and Sauce. Return the fried chicken pieces to the skillet, tossing to coat them evenly in the sauce. Cook for an additional 2-3 minutes until the sauce thickens and caramelizes.
Garnish and Serve. Transfer the sweet chili chicken to a serving dish. Garnish with sliced green onions and sesame seeds.