Mix 1/4 cup cornstarch, salt, and pepper together in a gallon-sized bag. Add the chicken pieces, seal the bag, and shake until the chicken is well coated.
Heat the oil in a large nonstick skillet over medium-high heat until shimmering, about 3 minutes. Add the chicken in batches, making sure not to crowd the pan, and cook about 2 minutes per side. The chicken might not brown, but the cornstarch coating will become crispy and dry. The chicken will not be completely cooked at this point, and that is OK. Transfer the chicken to a 6-quart slow cooker and repeat with any remaining chicken.
Whisk the sugar, rice vinegar, soy sauce, cider vinegar, sesame oil, 1 Tbsp cornstarch, garlic, ginger, and pepper flakes together in a small bowl. Pour over the chicken, then scatter the dried chile peppers around the chicken.
Cover and cook until chicken reaches 165F, 1 1/2 to 2 hours on the LOW setting or 45 minutes on the HIGH setting. Toss the cooked chicken with the sauce. Serve with rice. Enjoy!