Step 1: In a large mixing bowl, using an electric mixer (stand or hand-held) fitted with a paddle attachment, beat the softened unsalted butter on medium speed for 2-3 minutes until light and fluffy. Scrape down the sides of the bowl as needed.
Step 2: Gradually add the sifted powdered sugar, about half a cup at a time, beating on low speed until combined, then increasing to medium-high. Continue until all the sugar is incorporated and the mixture is crumbly. Scrape down the sides.
Step 3: Add the vanilla extract, ground cinnamon, and a pinch of salt (if using). Begin adding milk or heavy cream, one tablespoon at a time, beating on medium-high speed until the frosting reaches your desired consistency – light, fluffy, and spreadable. You might need slightly more or less liquid depending on humidity and how soft your butter was.
Step 4: Once all ingredients are combined, beat the cinnamon buttercream frosting on high speed for an additional 2-3 minutes. This step is crucial for incorporating air, making the frosting extra light and fluffy. Taste and adjust cinnamon or sweetness if desired.
Step 5: Use immediately or transfer the cinnamon buttercream frosting to an airtight container. Store at room temperature for up to 2 days, or in the refrigerator for up to 1 week. If refrigerating, let it come to room temperature and re-whip briefly before using.