Preheat the oven to 350. Line a 9 x 9 baking dish with parchment or spray with cooking spray, and set aside.
In a large bowl, stir together the butter, sugar, and brown sugar, until combined. Mix in the egg and vanilla until well blended.
In a small bowl, whisk together the salt, cream of tartar, cinnamon, and flour. Pour the dry ingredients into the wet ingredients, and mix until combined. Spread the mixture evenly in the prepared pan.
Whisk together the cinnamon and sugar, then sprinkle the mixture over the top of the batter, covering completely. Bake 24-26 minutes, or until a toothpick inserted in the center comes out clean. Let cool, then cut into bars.