PLACE chocolate and heavy whipping cream in small microwavable bowl, microwave on high for 30-second intervals, stirring well after each interval. (approximately 1 minute total cooking time) Let cool for 30 minutes.
BEAT butter and chocolate mixture with mixer until smooth in large bowl. Add vanilla extract and milk. Gradually add sugar beating well until all is blended. If needed, add more milk to achieve correct spreading consistency. Once smooth, do not continue to beat.
FROST cake immediately.