Preheat the oven to 350 degrees. Spray a 9x13 brownie pan with nonstick baking spray. Melt Butter and allow to slightly cool.
Pour the cake mix into a large mixing bowl. Add cream of tartar ¼ teaspoon of cinnamon and whisk well to combine.
Add melted butter, eggs, and vanilla to the dry ingredients. Stir with a rubber spatula or large spoon until the mixture comes together to form a dough.
In a separate, small bowl, combine sugar and cinnamon until fully combined.
Add 3 tablespoons of cinnamon sugar mixture into the cookie dough. With a rubber spatula, fold into the dough just barely. You should still be able to see the sugar mixture in the dough.
Spread the cookie dough evenly across the prepared pan. It will be relatively thin throughout.
Sprinkle the remainder of the cinnamon sugar on the top of the cookie dough.
Bake for 14-16 minutes until the edges are slightly brown and top of cookie bar is golden.
Allow cookies to cool in the pan for at least 15 minutes or until fully cooled. Cut, serve, and enjoy!