Pat the chicken thighs dry and place in a bowl. Add the cornstarch, salt, and ground pepper and toss until each piece is evenly coated.
In a small bowl, whisk together the chicken stock, honey, soy sauce, rice vinegar, red chili flakes, and chopped parsley; set the sauce aside.
Heat a skillet over medium heat and add the oil. When hot, add the coated chicken thighs in a single layer and cook 5–6 minutes per side, flipping once, until golden-brown and nearly cooked through.
Push the chicken to the side of the skillet and add the butter to the empty space. When the butter melts, add the minced garlic and cook about 1 minute until fragrant, stirring so it does not burn.
Pour the prepared sauce into the skillet, reduce heat to medium-low, and simmer 4–5 minutes, spooning the sauce over the chicken, until the sauce thickens and the chicken is cooked through.
Remove from heat and serve the honey garlic chicken hot.