In order to do this you must beat up the
yolks separately in a basin and add the hot butter sauce gradually,
otherwise the yolks of eggs will curdle and the sauce will be spoilt. In
fact, it must be treated exactly like custard, and in warming up the sauce
it is often a good plan, if you have no _bain-marie_, to put the sauce in a
jug and place the jug in a saucepan of boiling water. The sauce should be
flavoured with a little essence of mushroom if possible. Essence of
mushroom can be made from the trimmings of mushrooms, but mushroom ketchup