Chicken thighs coated in cornstarch and pan-fried form a different kind of crust than chicken dredged in flour. Flour makes a crust that softens over time. Cornstarch makes a glassy, shatter-when-you-bite-it crust that holds up against a sauce without turning soggy. If you've ever wondered why Chinese restaurant fried chicken has that specific crunch, cornstarch is half the answer.
Coat the chicken thoroughly. Pat the thighs dry, season with salt and pepper, then toss in cornstarch until every surface looks dry and chalky. Wet spots show up as pale patches after frying; dry-looking thighs emerge uniformly golden.
Pan-fry in a thin layer of oil over medium-high. Don't crowd, press down gently for full contact with the pan, and leave them completely alone for 4 to 5 minutes per side. Flipping early tears the crust off and leaves it in the pan. When the crust releases cleanly from the metal, that's the cue it's ready to flip.
Pull the chicken and build the sauce in the same pan. The fond left behind, those little crispy brown bits fused to the bottom, is a flavor foundation you can't buy. Garlic goes in first, scraping up the fond as it cooks. Soy, oyster sauce, sesame oil, chili crisp, and sugar follow. Thirty seconds of sizzle.
Egg noodles and chicken stock go in directly. Press the noodles into the liquid, bring to a simmer, and toss continuously as the liquid reduces and the noodles absorb the sauce. 3 to 4 minutes and the sauce is glossy and clinging to every strand. Slice the crispy chicken over the top. Scallions, sesame, extra chili crisp. The crust stays crunchy even on top of saucy noodles, which is the whole point.