NSTRUCTIONS In food processor, process butter, sugar, cocoa, and salt until smooth, about 30 seconds, scraping sides of bowl as needed. Add corn syrup and vanilla and process until just combined, 5 to 10 seconds. Scrape sides of bowl, then add chocolate and pulse until smooth and creamy, 10 to 15 seconds. Frosting can be used immediately or held (see note). Makes 3 cups to frost one 9-inch 2-layer cake 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.