Beat butter in the bowl of a stand mixer (or with handheld electric mixer) until fluffy and creamy (about 2 minutes). Add in powdered sugar and beat on low (so as not to make a huge mess!).
Add vanilla and 2 tsp. of milk. Keep mixing and adding little bits of milk until it reaches your desired consistency. Add cinnamon and beat until evenly distributed.
Frost cake once it's completely cooled, cut in squares and you're good to go!