Pour about ½ cup of chocolate onto a flat marble, glass, or stainless steel surface. Spread it into a 1-millimeter-thick rectangle, about 2 feet long by 6 inches wide. Allow the chocolate to set for 5 minutes, or until it is no longer liquid or shiny yet is still slightly soft. With a straight-edged stainless steel spatula held at a 45-degree angle, scrape the chocolate widthwise to form wavy ribbons. Transfer the ribbons to a tray and repeat the process until all the chocolate is used. Decorate the cake with the ribbons.