This recipe is so delicious that people ask to have more plum pudding just so that they can have an excuse to eat lots of sauce. This makes a large quantity but the base will keep for several weeks in the fridge, so you can use a little at a time, adding whipped cream to taste.
Melt the butter, stir in the sugar and allow it to cool slightly. Whisk the egg and add to the butter and sugar with the sherry and port. Refrigerate.
When needed, add the lightly-whipped cream to taste.
This sauce is also very good with mince pies and other tarts.