Add the softened butter to the bowl of a stand mixer and beat using the whisk attachment until light and fluffy.
With the mixer off, add the vanilla extract, half of the powdered sugar, and half of the milk. Whip until light and fluffy again, then add the rest of the powdered sugar and the rest of the milk one tablespoon at a time until you acheive the desired consistency.
Store in an airtight container in the fridge for up to 1 week. When you're ready to use it again, allow the buttercream to come to room temperature and then rewhip before using. Enjoy!
Notes
Storing instructions: store in an airtight container in the fridge for up to 1 week, or in the freezer for up to 2 months. To thaw, transfer to the fridge overnight.