Preheat the oven to 350 degrees. Grease a 13x9" baking dish with cooking spray.

In a medium bowl, whisk together the flour, baking soda, cream of tartar and salt. Set aside.

In a large bowl, beat the butter and the sugars until light and fluffy, about 2-3 minutes. Add the eggs one at a time, beating on low speed until incorporated. Beat in the vanilla. Beat in the dry ingredients on low speed until just combined.

Pour half of the batter into the prepared baking dish. The batter will be sticky, but spread it as evenly as you can. Sprinkle half of the cinnamon-sugar mixture over the batter.

Using a teaspoon, drop dollops of the remaining batter over the cinnamon-sugar mixture. The cinnamon-sugar mixture will still be visible in many areas. Sprinkle with the remaining cinnamon-sugar.

Bake for 25 minutes on the center rack of the oven. Remove from heat and cool on a wire rack for 1 hour. Cut into squares and enjoy!