Velvety Cocoa Frosting
["3 c. confectioners sugar", "1/2 to 3/4 c. cocoa", "1/2 c. butter, softened", "1/4 c. water", "1 egg yolk", "1 tsp. vanilla"]
["In large bowl with mixer at low speed, beat confectioners sugar, cocoa, butter, water, vanilla and egg yolk until smooth, occasionally scraping bowl with rubber spatula."]
["confectioners sugar", "cocoa", "butter", "water", "egg yolk", "vanilla"]