Whip the butter alone in a stand mixer on medium for 2 minutes or until fluffy, almost white in appearance and lighter in texture.
Add the vanilla and cinnamon. Add the powdered sugar slowly, mixing between each addition.
Alternate the last bit of powdered sugar and the heavy cream, and whip the frosting on high for at least 2 minutes. If the buttercream is too thick after 3 tablespoons of heavy cream, add a fourth tablespoon.
Frost cupcakes or cake with this buttercream, or store in an airtight container. This frosting must be stored in the fridge, and will keep for about 5 days. You can also freeze it for up to a month.
This is enough frosting to frost one 9x13 cake or one two layer 8 or 9" cake.