Preheat oven to 350°F
Butter or spray with non-stick spray a 8 x 8 baking dish
In a small bowl, combine flour, cream of tartar, baking soda, and salt. Set aside
Combine butter and sugars and beat on high in an electric mixer for 4-5 minutes, until light and fluffy
Gradually beat in eggs and vanilla into sugar/butter mixture until combined.
Add in dry ingredients and beat just until combined
Spoon half of the mixture into prepared pan, smoothing it out as much as possible
Sprinkle 75% of the cinnamon-sugar mixture over batter
Dollop remaining batter over the cinnamon-sugar mixture
Sprinkle the remaining cinnamon-sugar over the dough
Bake 40- 45 minutes or until a toothpick inserted in the middle comes out clean.
Cool completely for 1 hour before cutting.