Put the softened butter in a large bowl and beat it on medium speed for about 2 minutes, until it looks smooth and pale.
Add 4 cups of powdered sugar, then pour in the milk and vanilla extract.
Mix on low until the sugar starts to blend in.
Turn the mixer up to medium-high and beat for 2 to 3 minutes, until the mixture is creamy and thick.
Add the rest of the powdered sugar, about 1/2 cup at a time, beating well after each addition until you like the consistency.
Toss in a pinch of salt and beat on high for 3 to 5 minutes.
This gives you a fluffy texture that’s great for spreading or piping.
Scrape down the bowl, then mix for another 30 seconds to smooth everything out before you use it.