01 -
Use a stand mixer with a whisk on medium-high. Mix the butter until it's really fluffy and airy, about 3 minutes.
02 -
Pour in the vanilla extract and blend it with the butter.
03 -
Keep the mixer on low. Slowly add the sifted sugar, the milk, and the salt. Stop often to scrape the sides and bottom of the bowl.
04 -
Turn up to medium-high or high speed and whip for at least 3 minutes. You can go for up to 7 minutes if needed.
05 -
If it's too thick, beat in small amounts of extra milk until it's spreadable.
06 -
Put it in the fridge for up to two weeks. Whip it again before using.