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 50-55 minutes or until a toothpick inserted in the middle comes out clean.

Cool completely for 1 hour before cutting.