Put the chocolate in a large mixing bowl. Pour the cream into a saucepan with the sugar and bring to a simmer, stirring continuously until the sugar is melted. Bring to the boil, then quickly remove from the heat, pour over the chocolate and whisk until smooth and glossy. Use straight away or leave to cool down a little until thick enough to spread on a cake. Alternatively, leave to cool completely, then beat with an electric whisk until light and fluffy and spread on a cake.