Whisk yolks, eggs, cream, sugar & cornstarch in the top of a double boiler set over simmering water. Cook, stirring with a wooden spoon, until custard is thick enough to coat the back of spoon; remove from heat. Whisk in butter, vanilla & lemon zest. Strain the custard into a bowl. Place a piece of waxed paper directly over the custard; refrigerate until chilled or up to a day.