I found these two recipes online... I've never tried them. The second one sounds close to what you might be looking for. Good luck!
Favorite Hot Fudge Sauce
2 1/2 cups powdered sugar
1 cup heavy cream
1/2 cup butter
6 (1-ounce) squares baking chocolate
1/8 teaspoon salt
2 teaspoons vanilla extract
In the top of a double boiler combine powdered sugar, cream, butter, baking chocolate, and salt. Cook and stir over simmering water until melted.
Cover and cook 30 minutes over low heat, stirring frequently. Stir in vanilla. Remove from heat. Serve warm.
Makes about 2 cups.
Heavenly Hot Fudge Sauce
4 (1-ounce) unsweetened chocolate squares
1/2 cup butter
3 cups sugar
1 (12-ounce) can evaporated milk
1/2 teaspoon salt
Melt chocolate and butter in a large, heavy saucepan over low heat, stirring constantly. Add sugar, 1 cup at a time, alternately with evaporated milk, beginning and ending with sugar; stir constantly over medium heat 5 minutes or until smooth. Stir in salt. Serve warm or spoon sauce into hot sterilized jars, and seal. Store in refrigerator up to 1 month.
Makes about 4 cups.