Pound the chicken. Place the chicken thighs in a single layer between two sheets of plastic wrap on a cutting board. Pound with a meat mallet until 1/8 to 1/4 inch thick.
Set up a breading station. Add the flour to one shallow bowl, the beaten egg to a second, and the breadcrumbs mixed with salt and pepper to a third.
Bread the chicken. Dredge each thigh in flour (shake off excess), dip into the egg (let excess drip off), then press firmly into the breadcrumb mixture to coat both sides.
Prep the basket. Spray the air fryer basket with cooking spray and arrange the breaded thighs in a single layer without crowding.
Spray generously. Coat the tops of the breaded thighs evenly and generously with cooking spray so every spot of breading is covered — this prevents dry, white patches.
Air fry. Cook at 375°F for 12 minutes, flipping halfway through and spraying the second side with cooking spray.
Check and serve. The schnitzel is done when the crust is deep golden brown and the internal temperature reaches 165°F. Plate, serve, and enjoy immediately.