Combine powdered sugar, butter and cream in heavy medium saucepan. Stir over medium-low heat until smooth. Remove from heat. Add chocolates and stir until melted and smooth. Stir in vanilla extract. (Can be prepared 1 week ahead. Cover and chill. Rewarm over low heat before serving.)
Bon Appetit/Sept/94 Scanned & edited by Di Pahl & <gg>