Pat the boneless skinless chicken thighs very dry with paper towels. In a medium bowl, combine the cornstarch, salt, and black pepper. Add the chicken and toss until each piece is lightly and evenly coated.
Heat the vegetable oil in a large skillet (about 12 inches) over medium-high heat. When the oil is hot and shimmering, add the chicken thighs in a single layer. Cook for 4–5 minutes without moving them, until the bottoms are deep golden brown.
Flip the chicken thighs and cook for another 3–4 minutes, or until they are browned and mostly cooked through. The internal temperature should be close to 165°F (74°C) in the thickest part. If your pieces are very thick, you may need an extra 1–2 minutes.
Reduce the heat to medium. Push the chicken to one side of the skillet. Add the butter to the empty side of the pan. Once melted, add the minced garlic and cook for 30–60 seconds, stirring constantly, until fragrant but not browned.
In a small bowl or measuring cup, whisk together the honey, chicken stock, rice vinegar, and light soy sauce. Pour this mixture into the skillet, stirring it into the garlic and around the chicken thighs.
Bring the sauce to a gentle boil, then reduce the heat to medium-low. Let it simmer for 4–6 minutes, turning the chicken occasionally, until the sauce has reduced and thickened. The glaze should look glossy and coat the back of a spoon; if you drag a spatula across the pan, the sauce should briefly leave a trail before flowing back.
Check that the chicken has reached an internal temperature of 165°F (74°C). If needed, simmer for another minute or two, adding a splash of stock or water if the sauce becomes too thick or sticky.
Remove from the heat. Spoon the honey garlic sauce over the chicken thighs. Serve hot over rice, noodles, or steamed vegetables, and garnish with chopped parsley or green onions, red pepper flakes, and sesame seeds if desired.