Pat chicken thighs dry. Season generously with salt and pepper. Coat all sides evenly in cornstarch. It should look dry and chalky with no wet spots.
Heat oil over medium-high in a wide skillet. Add chicken and press down gently for full contact. Cook undisturbed 4-5 minutes until deeply golden and the crust releases cleanly. Flip and cook 3-4 more minutes. Remove to the side.
Sauce
Leave 1 tbsp oil in the pan. Add garlic and cook 20-30 seconds until fragrant, scraping up the fond. Add soy sauce, oyster sauce, sesame oil, chili crisp, and sugar. Stir to combine.
Noodles
Add noodles and stock to the pan. Press noodles down and bring to a simmer. Toss continuously as liquid reduces and noodles absorb the sauce, 3-4 minutes. Add a splash of water if it tightens too fast. Noodles should look glossy and clingy, not soupy.
Plate
Slice chicken thighs at an angle to show the golden crust cross-section. Plate noodles and lay chicken on top. Finish with scallions, sesame seeds, and extra chili crisp.