Use a paddle attachment on your mixer if you're using a stand mixer. If you don't have a paddle attachment, just use the whisk attachment will work fine. If you don't have a stand mixer, a handheld mixer will work just fine. Cream the butter well for a couple minutes on medium speed.
Add in 3 cups powdered sugar. Beat on low until combined, then on medium until well incorporated.
Add 1-2 tablespoons of milk, plus the last 3 cups of powdered sugar. Beat on low until combined, then on medium until well incorporated.
Add vanilla paste (or extract) and cinnamon. If the buttercream is very thick, add another tablespoon of milk.
Beat on low until combined and then on medium until well incorporated.
Check the consistency. If it's too thick, add milk (a tablespoon at a time) and mix after each addition until you reach the desired consistency.
This buttercream is fine to keep out of the refrigerator for a couple days. After that, it can be kept in the fridge for about a week.This recipe card may contain affiliate links. As an Amazon Associate, I earn from qualifying purchases.