In a medium microwavable bowl, combine 1 tablespoon unsalted butter or regular butter and 1/2 ounce unsweetened chocolate, cut up. Microwave on (high) for 45 to 60 seconds or until the chocolate is softened (or heat chocolate in a saucepan over low heat).
Stir until smooth; cool to room temperature.
Stir in 1/2 teaspoon vanilla and dash salt.
With electric mixer on low speed, beat in 3/4 cup sifted powdered sugar and 1 to 2 tablespoons milk; beat on high speed until desired consistency. Spread over top of cake.