Melt 2 squares chocolate in top of double boiler. When melted, add 6 tablespoons light cream and 3 tablespoons butter; stir until smooth and slightly thick. Remove from heat; add 1 pound (4 1/2 cups) sifted confectioners' sugar, all at once, 1/8 teaspoon salt and 1 teaspoon vanilla. Beat. If too thick to spread, thin with a few drops of cream. Frosts two 8 or 9-inch layers.