Melt the chocolate with the butter in a heatproof bowl over simmering water. Remove from the heat and beat in the confectioners' sugar.
Whip the cream in a separate bowl until it just holds its shape, then beat into the chocolate mixture. Allow the mixture to cool, stirring occasionally, until it begins to hold its shape. Use immediately.
VARIATION: Stir in a tablespoon of brandy or your favorite liqueur.