Melt two 1 ounce squares unsweetened chocolate and 3 tablespoons butter over hot water.
Remove from heat.
Stir in 3 cups sifted confectioners' sugar, clash salt, and 1 1/2 teaspoons vanilla.
Gradually stir in 5 to 6 tablespoons boiling water to make thin frosting. (If frosting begins to thicken, add additional boiling water.)
Fill and frost Viennese Torte, as directed.