In a saucepan off the heat, combine the sugar, cornstarch and zest. Whisk in the eggs, egg yolks, water and lemon juice.
Over medium heat, cook while stirring constantly making sure to scrape the bottom and sides of the pan, until the curd thickens or a candy thermometer reads 170°F (76°C). Remove from the heat.
Strain the curd through a fine sieve. Add the butter and stir until melted and smooth. Pour into a bowl and cover with plastic wrap directly on the surface of the curd to prevent a skin from forming. Let cool. Refrigerate for 2 hours or until completely chilled.
The curd keeps up to 1 week in the refrigerator.
This basic lemon curd is perfect in layer cakes, as an ice cream topping or a tart filling.