Microwave the butter in a microwave safe bowl just until melted. Stir in the semi-sweet and unsweetened chocolate. Microwave in 20 second intervals, stirring between each interval, until the chocolate is melted.
In a large mixing bowl, combine the melted chocolate and butter mixture, powdered sugar, sour cream, and vanilla; beat with an electric mixer for 3-4 minutes or until smooth.
Spread the frosting evenly onto a cooled cake or cupcakes.
Refrigerate any leftover frosting.
The calories shown are based on the frosting going on a cake that serves 15 people, with 1 serving being 1/15 of the frosting recipe. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.