This is a very old English recipe used for a spread on toast and a filling for tarts. My mother used it as a filling in layer cakes. Makes about three cups.
4 large juicy lemons
2 cups sugar
1/2 cup unsalted butter
4 egg yolks, well beaten
V.S.P. Scrub the lemons, grate the rind, cut in halves and squeeze out the juice. Pour the juice into the top of a double boiler, add sugar, butter and grated rind. Cook over medium heat until the sugar dissolves. Pour this mixture over the egg yolks, stirring all the while to prevent curdling. Return the mixture to the pan and continue cooking over gentle heat until the mixture thickens. Put into sterilized jars and seal at once. Cool before serving.