Mustard Butter This page first published: Jan 25, 2006 · Updated: Jun 23, 2018 · by CooksInfo. Copyright © 2021 · This post may contain affiliate links · This blog generates income via ads · Mustard butter is softened butter with mustard blended in. Cooking Tips Proportions: per 200 g (7 oz / 1 cup minus 2 tbsp) butter, 2 tablespoons mustard.