Cambridge Butter

Lay in a mortar and pound well, six hard boiled egg-yolks, with four well cleansed anchovies and a spoonful of chopped capers, also some tarragon and chives; when the whole is reduced to a paste, add to it one spoonful of English mustard, and the same quantity of French mustard, some salt, pepper, and vinegar, and one pound of fresh butter. Rub the compound through a sieve, and then mix in with it a tablespoonful of chopped parsley.