Grate 1 teaspoon rind from a lemon. Cut lemons in half; squeeze, reserving 3 tablespoons juice.
Combine sugar and cornstarch in a heavy nonaluminum saucepan. Add lemon juice, salt, yolk, and 3/4 cup water; cook over medium heat, stirring constantly, until mixture boils. Boil, stirring constantly, 1 minute. Remove from heat. Stir in rind and butter. Serve in tart shells or use as a frosting.
NOTE: To frost top and sides of a 2-layer (9-inch) cake, use 1/3 cup lemon juice and 3 egg yolks, and double remaining ingredients except 1 tablespoon butter.