Preheat oven to 350 F and line pans with cupcake liners.
In small bowl, sift cake mix and cinnamon; set aside.
In large bowl, combine eggs, oil, buttermilk, and vanilla extract.
Add cake mix and stir until smooth.
Fold in grated zucchini.
Fill cupcake liners ¾ full and bake for 15-20 minutes or until knife inserted near center comes out clean.
For buttercream, beat butter and cream cheese for 2 minutes.
Scrape down bowl and beat again.
Add cinnamon and slowly add powdered sugar until buttercream reaches desired consistency.
Add milk if buttercream becomes too thick.
Pipe buttercream onto cooled cupcakes.
Top with cinnamon and sugar.