Season thighs with salt and pepper. Brown in a large skillet with olive oil, skin side down, 5–7 minutes; flip and cook 3 minutes. Remove and set aside.
Reduce heat, add garlic to skillet and sauté 30 seconds until fragrant. Stir in honey, soy sauce, vinegar and butter.
Return thighs to skillet, spoon sauce over, cover and simmer 18–22 minutes until cooked through. Remove thighs and keep warm.
Whisk cornstarch slurry into sauce, simmer 1–2 minutes until thickened. Return thighs to coat.
Boil potatoes until tender, drain, mash with milk and butter, season with salt. Plate mashed potatoes and spoon chicken and sauce over top. Garnish if desired.