Preheat oven to 350 degrees. Spray desired cake pan with non-stick cooking spray. Prepare box cake mix as directed on package. Add 2 teaspoons of cinnamon to cake batter if desired. Pour cake mix into desired pan and bake as directed on the box. Remove cake from oven and let cool completely.
To make dulce de leche buttercream, with a hand or stand mixer, combine the softened butter with the powdered sugar. After the two are well incorporated add the can of prepared dulce de leche and vanilla. Add the heavy cream starting with 1 tbsp until desired spreading consistancy is achieved. After baking let cake cool completely before adding dulce de leche buttercream.