Place a chicken breast in between two pieces of cling wrap. Use a mallet or rolling pin to pound out until 2cm thick. Repeat with remaining chicken.
Combine breadcrumbs and parmesan in a medium bowl. Whisk together the egg and milk in a separate medium bowl. Place the flour on a plate and season.
Dip the chicken in the flour, shaking off excess. Dip in egg, then breadcrumb mixture, pressing firmly to coat. Transfer to a tray. Cover and place in the fridge for 15 minutes.
Preheat the 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 basket of the air fryer. Set the timer for 12 minutes. Cook, turning chicken halfway through, until golden and cooked through. Transfer to a plate and cover with foil to keep warm. Repeat with the remaining chicken.
Serve the schnitzel with lemon wedges and Darlington Farms BBQ Classic Sauce.