Coat the inside of a large saucepan with the melted butter. Then put in the plums and cook over low heat, stirring constantly with a large wooden spoon, for about 20 minutes, or until they are soft.
Put the plums through a strainer or food mill, clean the saucepan, and return the plum pulp to it. Stirring all the time, cook over low heat until the pulp is very thick and dark. Stir in the grated peel, if using, and the sugar. Put the butter into jars, cover and process. To make about 7 pints.