In a large stand mixing bowl, mix butter on a medium speed until fluffy.
Slowly add in sugar. **Make sure the sugar is sifted!! I made this mistake once and you will not get smooth icing without sifted sugar**
Continue mixing until well blended.
Add vanilla. Mix well.
Slowly add in cream or milk. Blend on low speed until mixed through.
Beat at high speed until frosting is smooth and fluffy.