Bread pudding sauce

1. Half-fill the lower half of a double boiler with water and bring to a boil.
2. Meanwhile, cream together sugar and egg yolk (in a food processor, or beating in a bowl with a wooden spoon).
3. Add cornstarch and salt and blend well.
4. Add 300 ml water a little at a time, blending after each addition, until all water is absorbed.
5. Pour mixture into the upper half of the double boiler and stir constantly and strongly over low to medium heat, scraping the bottom of the pot, until mixture begins to thicken.
6. (if egg starts to scramble, remove the top half of the double boiler for a moment and stir fiercely to cool the mixture, then return it to reduced heat).
7. Add the butter and lemon juice and continue stirring until butter melts and mixture is creamy (about 10 minutes more).
8. If there are any lumps, force sauce through a strainer.
9. Serve hot or warm.
10. sh "hard sauce" substitute 50 ml brandy, whiskey, or rum for the lemon juice.
11. Difficulty : easy.
12. Precision : measure the ingredients.