At low speed, add in the powdered sugar and cinnamon until mostly incorporated. Turn the mixer up and beat until creamy. You may need to scrape down the sides of the bowl and mix again.
Store tightly covered in the refrigerator.
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Notes
Be sure to allow your unsalted butter to fully soften before beginning. This will make it much easier to mix all the ingredients together leaving you with a creamy spread. Cut the butter into small cubes and allow it to rest at room temperature for around 20 minutes. The smaller you cut the butter into cubes, the quicker it'll come to room temperature. If the butter isn't softened to room temperature, you'll end up with a clumpy mixture.