From: The Thorough Good Cook

Sauces 7. Béarnaise

Mix half a teaspoonful of flour with a little butter. Moisten with mushroom juice, and let it boil for a few minutes; thicken with the yolks of one or two eggs beaten up in cream. Add a few mushrooms, truffles chopped up fine, some essence of anchovy, and the juice of a lemon.