Start by making the crust. In a large bowl, mix the flour, powdered sugar, cornstarch, and salt. Then add the butter mix well until the mixture is coarse crumbs.
Spread the mixture into a sprayed pan and press firmly into an even layer over the pan's bottom and slightly up on the sides. Place into the refrigerator for about 20-30 minutes until firmed up. Then place into the air fryer for 15-20 minutes at 320 degrees F, air fryer setting, until golden brown.
Making The Lemon Filling: While the shortbread is cooking, make the lemon filling in a large bowl, mix the eggs, sugar, flour, mix in the lemon zest, lemon juice, milk, and salt.
Place the filling into the crust, bake until the filling is firm to the touch. About 10-12 minutes at 300 degrees F, air fryer setting.
Sprinkle with powdered sugar right before serving.
Plate, serve, and enjoy!