Preheat oven to 350 degrees. Butter a 9-by-13-inch glass baking dish; line dish with waxed paper.
Make crust: An adult should grate butter on a cheese grater with large holes. In a separate large bowl, whisk together flour, confectioners' sugar, and salt. Add butter; stir with a wooden spoon until combined and mixture looks crumbly.
Transfer mixture to baking dish; press evenly onto bottom with your hands. Chill crust in freezer 15 minutes. Bake until slightly golden, 16 to 18 minutes.
Meanwhile, make filling: In a large (1 quart) liquid measuring cup, whisk together eggs, sugar, flour, and salt until smooth. Stir in lemon juice and milk.
An adult should remove crust from the oven and pour filling mixture over hot crust. Reduce oven temperature to 325 degrees and bake until filling is set and edges are slightly golden brown, about 18 minutes. An adult should remove dish from the oven. Let cool completely on a wire rack.
Grip paper; lift out dessert, and set on cutting board. With a sieve, dust with confectioners' sugar. Cut into squares.
The filling for these lemon squares is very tangy; if you want a less tart version, use 1/2 cup lemon juice and 1/2 cup milk.