Your butter should be room temperature and soft to the touch.
Cream the butter for a couple of minutes, it's worth it to get the it extra creamy.
Add the powdered sugar a little bit at a time and it will come together easier.
Use immediately on cooled cookies.
How to Store
Room Temperature – If you’re using this buttercream on frosted cookies, store them in an airtight container for up to three days.
Refrigerate – Refrigerate your buttercream for up to one week in an airtight container.
Freezer – Freeze buttercream frosting for three months in an airtight container. Before using, move it to the refrigerator overnight and bring to room temperature before using.