The butter should be soft, but not too soft that it has started to melt.
With the paddle attachment fitted, beat butter on medium speed of your stand mixer, about 30 seconds.
With the mixer running on low speed, gradually add the sugar, 1 cup at a time, until incorporated. (To avoid a poof of sugar, turn your mixer off each time you add a cup of sugar.)
Turn the mixer up to medium speed and beat for about 2 minutes. Stop the mixer and scrape down the side and bottom of the bowl with a rubber spatula.
Add the vanilla and 2 TBSP of heavy cream, then beat on medium speed for 2-6 minutes, until the frosting is light and fluffy.
Use immediately to frost cakes and cupcakes.