Mushroom Butter This page first published: Jan 25, 2006 · Updated: Oct 4, 2020 · by CooksInfo. Copyright © 2021 · This post may contain affiliate links · This blog generates income via ads · Mushroom Butter is butter blended with an equal weight of puréed fresh mushrooms seasoned with salt and pepper, then pressed through a sieve. It is used as a garnish for cold hors d’oeurvres, and to flavour white sauces.