In a food processor, combine the flour, sugar, cornstarch, and salt. While the motor is running, add the butter and continue to process until mealy. Transfer mixture onto a buttered 12 by 18-inch sheet tray. Press to form an even layer of crust.
Bake for 15 to 18 minutes or until light golden in color.
Meanwhile, prepare the lemon topping. In a medium bowl, whisk the eggs. Add the sugar, flour, and salt. Stir until well blended. Lastly, stir in the lemon zest, juice, and milk.
Pour over the prebaked crust and continue to bake another 20 minutes or until set. Chill for 2 hours before cutting into bars. Dust with powdered sugar.