Sift together (see recipe notes) powder sugar, unsweetened cocoa, and cream of tarter into a large bowl. Stir in milk until well incorporated. Cover bowl and refrigerate for 30 minutes, or until well chilled.
Using an electric or stand mixer, gradually beat whipping cream into chocolate mixtures, scraping down bowl to make sure all chocolate is mixed in. Continue mixing on high until very stiff peaks form. Makes 5 cups.
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Tip: Sifting together dry ingredients eliminates powder lumps in the frosting.