Add the butter to a mixing bowl if you are using a hand mixer or the bowl of a stand mixer. Beat on medium to high speed for about 5 minutes or until the butter has nearly doubled in volume and is light and fluffy.
Gradually add the sugar, half a cup at a time. Waiting until each measurement of sugar has been combined before adding the next.
Scrape down the sides of the bowl and add the vanilla. Add the milk if needed. Mix until combined.
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Notes
Only add the milk gradually if you would like to thin out the buttercream. You may want it thicker for piping but thinner for spreading over a cake.The buttercream will keep for 3 days at room temperature, 1 week in the fridge and 3 months in the freezer. If you store it in the fridge or freezer let it come up to room temperature and give it a good mix before using.