In large mixing bowl, beat butter until fluffy. Add three cups sugar, cocoa powder, 1 tablespoon milk, and vanilla extract.
Beat until combined and fluffy.
Adjust with more sugar or milk, depending on what consistency you're looking for.
Notes
Storage: There is a little bit of milk in this frosting recipe but it will be fine at room temperature for a few days. Any longer and it should be refrigerated.
Make Ahead: It can last in your refrigerator for about 1 week. But it can also be frozen up to 3 months. When ready to use, just thaw in your refrigerator and then re-whip back to its original consistency.