Make a chocolate strip to wrap around the cake

Buy a piece of flexible acetate at a baking-supply store or hobby store. Make sure it is long enough to wrap around the cake. Cut it to the width you want the strip—the strip looks dramatic if it covers the cake and extends an inch or so above the top of the cake—and melt bittersweet chocolate. Using an offset spatula, spread the chocolate in a thin layer on the acetate, and then weight both ends of the acetate until the chocolate is cool. As soon as the chocolate loses its sheen, wrap the acetate, chocolate side in, around the cake and refrigerate for 15 minutes. Peel away the acetate.