Pat chicken thighs thoroughly dry with paper towels. Season both sides evenly with kosher salt, cracked black pepper, and garlic powder.

Heat olive oil in a large cast-iron skillet over medium-high heat. Place chicken thighs skin-side down and sear undisturbed for 8 to 10 minutes until the skin turns deep amber-golden and shatteringly crisp. Flip and cook 3 minutes on the reverse side; transfer to a plate.

Pour off all but 1 tablespoon of rendered chicken fat from the skillet. Melt in 1 tablespoon butter over medium heat.

Add sliced Honeycrisp apples and quartered shallots. Sauté for 4 to 5 minutes until caramelized around the edges but still firm in the center. Transfer to the plate with chicken.

Pour apple cider into the hot skillet, scraping up all browned fond from the bottom. Whisk in Dijon mustard, maple syrup, apple cider vinegar, and fresh rosemary sprigs.

Simmer vigorously over medium-high heat for 6 to 8 minutes until liquid reduces by half into a glossy, fragrant syrup. Whisk in remaining 1 tablespoon cold butter to create a silky emulsion.

Nestle chicken thighs (skin-side up so skin stays crisp), sautéed apples, and shallots back into the bubbling sauce. Simmer gently for 4 minutes until chicken reaches 175°F (79°C). Spoon sticky glaze generously over the apples and around chicken. Serve hot!