Whip whole eggs and sugar for 5 minutes until foamy.
Put the mixture in the top part of a double boiler and start cooking it over hot water. (If you are experienced, you can do this over direct heat, but don’t make scrambled eggs out of it.) Stir in lemon juice and rind.
Drop by drop add the melted butter while still stirring over low heat. Essentially this is the same method used in making a hollandaise, and cream should thicken similarly.