In a large bowl, beat the butter, cream cheese, and sugar together. Add in the cinnamon (if using), milk, and vanilla. Continue beating until the frosting is well mixed and creamy. If the frosting is too soft, add in additional powdered sugar, 1 tablespoon at a time, until it reaches desired thickness. If the frosting is too thick, add more milk, 1 teaspoon at a time, until it reaches the desired thickness.
¾ cup unsalted butter, ¼ cup cream cheese, 3½ cups powdered sugar, 2 tablespoons milk, 2 teaspoons pure vanilla extract, 1 teaspoon ground cinnamon