Wash and dry spinach leaves and pound them in a mortar. Put into a strong cloth, twist to extract all the juice; pour this into a double saucepan to thicken. Turn out onto a muslin cloth stretched over a bowl, and filter. Scrape off the green residue which remains on the cloth; this is called vert d’epinard (spinach green). Add double its weight of softened butter which has been rubbed through a sieve.