In a saucepan off the heat, whisk together the sugar, cornstarch and zest. Whisk in the eggs and egg yolks. Add the water and lemon juice and mix well. Bring to a boil over medium heat, whisking constantly and scraping the sides and bottom of the pan until the mixture thickens. Remove from the heat. Pass through a sieve to remove any lumps. Whisk in the butter until smooth.