In a small saucepan, cook and stir 1/3 cup heavy cream and 6 ounces finely chopped bittersweet or semisweet chocolate over low until chocolate melts. Remove from heat. Let cool about 20 minutes or until it slightly thickens. (Chocolate should be warm but not hot.)
Crème de Menth Filling
In a large mixing bowl, combine 1/3 cup softened butter and 3 tablespoons green crème de menthe or creme de menthe syrup. Gradually add 4 cups powdered sugar, beating with an electric mixer on medium until smooth. If necessary, add 1 tablespoon additional crème de menthe to make spreading consistency. (Or, if you like, substitute milk for green crème de menthe. Add 1 1/2 teaspoons vanilla, 1/2 teaspoon peppermint extract and 2 or 3 drops green food coloring.)
430 calories; fat 20g; cholesterol 78mg; saturated fat 12g; carbohydrates 62g; insoluble fiber 2g; protein 4g; vitamin a 534.5IU; sodium 339mg; calcium 101mg; iron 1.6mg.