Ingredients - Makes about 2 cups
1 stick unsalted butter (room temperature)
1 and 2/3 cups powdered sugar
2/3 cup evaporated milk
6 squares (6-ounces) unsweetened block chocolate
1/4 teaspoon salt
Steps

1. In a mixing bowl, cream butter and sugar until fairly smooth. 
2. Place butter mixture in the top of a double boiler, adding milk and chocolate.  Let simmer without stirring for 30 minutes.
3. Remove from heat and stir in salt until smooth.

​If you don't understand or want to bother with a double boiler, then you can use a saucepan and very, very low heat.