In a large bowl, Whip the butter on high speed, occasionally scraping down the sides of the bowl. Whip until double in volume. This takes a very long time, approximately 10 minutes.
While still whipping, drizzle in the heavy cream. Continue whipping until the mixture is stiff, approximately 2 to 4 minutes.
Add 1/2 of the confectioners sugar and whip until smooth. At this point, you can taste the frosting and determine if you would like to add more sugar to sweeten to taste.