Foolproof Chocolate Frosting

This frosting may be made with milk, semisweet, or bittersweet chocolate. For our Fluffy Yellow Layer Cake (see related recipe), we prefer a frosting made with milk chocolate. Cool the chocolate to between 85 and 100 degrees before adding it to the butter mixture. The frosting can be made 3 hours in advance. For longer storage, refrigerate the frosting, covered, and let it stand at room temperature for 1 hour before using.