Whisk together the lemon zest and juice, whole eggs, egg yolks, and sugar in a large, heavy saucepan. Cook over medium heat, stirring constantly with a wooden spoon, until the mixture thickens, 20 to 25 minutes.Advertisement
Remove from heat, and stir in the butter, one piece at a time, until melted. Strain the mixture through a fine-mesh sieve into a medium bowl. Cover with plastic wrap, pressing it directly on the surface. Refrigerate until chilled, up to 3 days.
The amounts provided for filling a standard two-layer 4-inch-tall tier with a 1/4-inch-thick coating of curd between the layers. For 6-inch: 1/2 cup to fill; 8-inch: 1 cup to fill; 10-inch: 1 1/2 cups to fill; 12-inch: 2 cups to fill. (Note: Many cake makers will split the two cake layers in half horizontally and spread filling between all four resulting levels; if this is your plan, triple this amount.)
Curd will keep in the refrigerator for up to 3 days.