Beat butter and cream cheese at medium speed with an electric mixer until creamy. Gradually add powdered sugar, beating at low speed until blended. Increase speed to medium, and slowly add milk and vanilla, beating until smooth.
Microwave dark chocolate morsels in a microwave-safe bowl at MEDIUM 1 1/2 to 2 minutes or until melted and smooth, stirring at 30-second intervals. Gradually add melted chocolate to butter mixture; beat until blended and smooth.
Note: For testing purposes only, we used Nestlé Chocolatier Dark Chocolate Morsels.