Combine the butter and shortening in a mixing bowl. Beat on medium speed with an electric mixer until creamed together. Sift the cocoa powder and confectioners' sugar together. With the mixer running, slowly add the cocoa mixture to the butter mixture. With the mixer running, add the milk one tablespoon at a time until the frosting is smooth. Do not add more than 3 tablespoons of milk or the end result will be too runny. Beat the fudge topping and vanilla extract into the frosting mixture until it is smooth. Use the chocolate fudge buttercream frosting immediately.