In the classical French-cooking version, to make Green Butter, spinach leaves are washed, dried, crushed, then juiced by twisting them in a cloth. The spinach juice is then reduced and thickened through simmering over a bain marie, then strained through filter paper or a cloth, and let cool. It is then blended into softened butter.
French Green Butter is used as a garnish for cold hors d’oeuvres, and as a flavouring for white sauces.
English versions of Green Butter will be butter mixed with chopped parsley, or with a combination of herbs such as parsley, spinach, mint and sage.
More modern versions appear to also use fresh marijuana leaves (sic.)
At Magdalen College, Oxford, a version is made from butter, anchovies and a good deal of parsley.