1. Preheat oven to 350ºF. Grease a 9-by-13-inch pan; line with parchment paper. Lay another sheet of parchment crosswise, leaving a 2-inch overhang on both sides.
2. Pulse flour, confectioners' sugar, cornstarch and salt in a food processor. Pulse in butter until mixture resembles coarse meal. Pulse in yolk until mixture comes together.
3. Press into an even 1/4-inch-thick layer in pan. Freeze for 15 minutes. Bake, rotating halfway through, until crust is lightly browned, about 20 minutes.
4. Make filling: Mix flour and sugar. Whisk in lemon zest and juice. Whisk eggs, yolk and salt into lemon mixture.
5. Pour lemon mixture over warm crust. Reduce oven temperature to 300ºF. Bake until filling is just set in center, about 30 minutes. Let cool completely in pan on a wire rack, then cover and refrigerate until cold, at least 1 hour. Dust with confectioners' sugar, if desired.