Step 1: Place the softened unsalted butter in the bowl of a stand mixer fitted with the paddle attachment (or use a large bowl and a handheld electric mixer).
Step 2: Beat the butter on medium-high speed for about 3-5 minutes until it is light, fluffy, and almost white in color.
Step 3: Reduce the mixer speed to low and gradually add 3 cups of the sifted powdered sugar, one cup at a time, mixing well after each addition.
Step 4: Add the ground cinnamon, vanilla extract, and a pinch of salt. Mix on low speed until combined.
Step 5: Add 2 tablespoons of heavy cream and increase the speed to medium. Beat for another 2 minutes. If the cinnamon buttercream frosting is too thick, add more cream one tablespoon at a time until the desired consistency is reached.
Step 6: If the frosting is too sweet or soft, add the remaining cup of powdered sugar. Once perfect, give it one final whip on medium-high for 30 seconds.