In a medium bowl, mix together the powdered sugar and coco powder.
In a large bowl, cream the butter and 1 cup of the powdered sugar/coco powder mixture. This can take a couple of minutes, even if it seems dry at first, it will eventually cream together.
After this has creamed, slowly add in the milk and the rest of the sugar/coco poweder mixture, alternating between the two until everything has creamed together.
If you plan on adding it to any baked goods, make sure that they are cooled off enough to where it won’t melt the frosting.