Put the butter into a large bowl and blend until fluffy, about two minutes, using a hand mixer.
Sift in the confectioners sugar one cup at a time, beating well after each addition.
Once the confectioners sugar is fully mixed into the butter, add in the milk or heavy cream and the vanilla. Use a spatula to spare down the sides of the bowl, and beat with the hand held mixer one last time, incorporating the milk, vanilla and all of the butter or confectioners sugar that was left on the sides of the bowl.
Once buttercream is done, you can pipe on to cupcakes, spread onto cakes, and much more. Just make sure that if you are putting it on something that has been in the oven, it is at least room temperature as it could melt and slide off of your dessert. Store in the fridge for up to two weeks.