1. Heat the cream in medium-size saucepan over medium-low heat until bubbles form around the inside edge of the pan.
2. Place the chocolate in a bowl. Pour the cream over the chocolate, and stir until the chocolate is melted. Whisk in the butter until the icing is smooth and well blended.
3. Spoon icing over cake, letting icing drizzle down sides so cake is completely covered with icing. (If icing begins to harden in bowl, gently reheat.) Refrigerate cakes until icing is set, before proceeding with decorating.