Put the butter and sugar into the top of a double boiler or basin over boiling water. Stir until the butter has melted. Blend the eggs with the grated zest, then strain over the juice. Add this to the butter and sugar and cook over hot water until the mixture thickens sufficiently to coat the back of a wooden spoon. Pour into hot sterilized jars, cover and store for up to 3 months.
To make orange curd, replace the lemons by oranges, but add the juice of ½ a lemon.