11 ounces (1 1/3 cups; 310g) lemon juice, from about 8 large lemons (see note above)
2 ounces (1/2 cup; 55g) powdered sugar (optional)
1 tsp citric acid
For the Crust: Adjust oven rack to middle position, preheat oven to 350°F, and line an 8- by 8- by 2-inch anodized-aluminium baking pan with a piece of parchment paper long enough to overhang the pan's edges. Combine flour, powdered sugar, salt, lemon zest, and butter in the bowl of a food processor; pulse to form a dry but pebbly meal, about twelve 1-second pulses. Scatter into the prepared baking pan, press into an even layer, and bake until pale gold and firm, about 30 minutes. Use hot or set aside until needed, up to 48 hours at room temperature if tightly covered.
For the Custard: Combine whole eggs, yolks, sugar, salt, and lemon zest in a 3-quart stainless steel saucepan, then whisk in lemon juice. Cook over low heat, stirring constantly but not vigorously, until warm to the touch, about 3 minutes. Increase heat to medium-low and continue cooking and stirring until thick and steaming-hot (custard should register 170°F on an instant-read thermometer), about 8 minutes. Immediately strain into the prepared crust, smooth into an even layer, and cover with aluminium foil. Bake until smooth and firm, about 10 minutes. Remove foil and cool to room temperature, then re-cover and refrigerate until cold and firm, about 1 hour.
To Serve: Loosen chilled custard from sides of pan with a bench or butter knife, then carefully tug loose and lift using overhanging parchment and transfer to a cutting board. Cut into 16 two-inch squares, rinsing the knife clean between each slice. Serve plain, lightly dusted with powdered sugar mixed with citric acid.