Using a copper bowl, blanch 90 g-100 g (3–3½ oz) of mixed leaves of watercress, parsley, chervil, tarragon and chives in equal quantities and 25 g (1 oz) spinach leaves; refresh, drain and squeeze out all the moisture. Separately, blanch 40 g (1½ oz) finely chopped shallot, drain and squeeze. Pound the herbs and shallot together finely.
Add 50 g (2 oz) gherkins, 1 tbs squeezed capers, 1 small clove of garlic and 8 fillets of anchovy and pound all together to a fine paste. Add 750 g (1 lb 10 oz) butter, 3 hard-boiled egg yolks and 2 raw egg yolks; mix in, then finally add 2 dl (7 fl oz or ⅞ U.S. cup) oil, drop by drop. Pass through a fine sieve, mix together with a whisk until smooth, season with salt and finish with a very small pinch of Cayenne.
This butter is used as an accompaniment for cold fish and it may be used to coat fish prepared for a cold buffet.