In a medium bowl, combine butter, confectioners’ sugar, cocoa, salt, vanilla extract, and milk. Beat at low speed with an electric mixer. Increase power to high speed, beating until all ingredients are incorporated and mixture is smooth and fluffy, 2 to 3 minutes, and scraping down sides of bowl while beating if necessary. Use immediately, or cover and store in refrigerator. Let come to room temperature before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chocolate-guinness-mini-cupcakes-recipe/