Place all ingredients into a bowl of a stand mixer or bowl where you will be mixing in.
By hand using the mixing beater, mix the butter and powdered sugar up a bit to avoid the powdered sugar from causing a mess when turning the mixer on.
Turn the mixer on at a slow setting to start with then gradually bring the speed up as the mixture comes together.
Scrape the sides and bottom of the bowl 2-3 times during the mixing process. The whole process will take about 7-10 minutes. Mixture should be smooth and not grainy. Any unmixed sugar will cause the mixture to to be grainy.
If needed, store in the refrigerator for up to 5 days or in the freezer for up to 3 months.
Notes
Try to start with a paddle attachment with your stand mixer. At the end, however, try switching to a whisk attachment. This will allow more air to be incorporated into the buttercream. It will make it fluffier.