- 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.