Beat first 4 ingredients at medium speed with an electric mixer until creamy.
Gradually add powdered sugar alternately with 1 Tbsp. milk, 1 Tbsp. at a time, beating at low speed until blended and smooth after each addition. Beat in finely chopped bittersweet chocolate baking bar.
Note: For testing purposes only, we used Ghirardelli 60% Cocoa Bittersweet Chocolate Baking Bar.