To make this Old-Fashioned Butter Mints Recipe gluten-free, use certified gluten-free powdered sugar, vanilla extract, peppermint extract, food coloring, and any packaged dairy ingredients. Butter, heavy cream, milk, and salt are typically gluten-free, but labels should be checked for additives or cross-contamination. Prepare the mints on a clean surface with utensils and bowls that have not come into contact with gluten-containing ingredients.