1 stick (4 ounces) cold unsalted butter, cut into tablespoons
In a small heavy saucepan, whisk together the eggs and egg yolks. Whisk in the sugar, then whisk in the citrus juices and zests. Add the butter and whisk constantly over moderately low heat until the mixture thickens, about 8 minutes; do not let it boil. Immediately strain the curd into a bowl. Press plastic wrap directly on the surface of the curd and cut 6 small steam vents in the plastic. Refrigerate the curd overnight; it can be refrigerated for up to 1 week.