Place 1 chicken breast in between 2 pieces of plastic wrap. Use a meat mallet or rolling pin to gently pound out until 2cm thick. Repeat with the remaining 3 chicken breasts.
Combine breadcrumbs and parmesan in a medium bowl. Whisk egg and milk in a separate medium bowl. Place flour on a plate and season.
Dip the chicken pieces in the flour mixture, shaking off the excess flour. Dip in egg, then breadcrumb mixture, pressing firmly to coat well. Transfer to a tray. Cover and place in the fridge for 15 minutes to firm up.
Preheat an air fryer to 200°C for 3 minutes. Spray the chicken with olive oil spray. Arrange half the chicken in a single layer in the air fryer basket. Cook, turning chicken halfway through, for 12 minutes or until golden and cooked through. Transfer to a plate and cover with foil to keep warm. Repeat with the remaining chicken.
Serve the chicken schnitzels with lemon wedges alongside.
