Preheat oven to 375 degrees.
Coat a 9x13-inch baking dish with nonstick spray.
Combine the 1/3 cup sugar and the 1 tsp. cinnamon. Set aside.
In a medium bowl, whisk together the flour, cream of tartar, and baking soda.
In a large bowl, cream the butter and 1 1/4 cups of sugar over medium speed until light and fluffy (about 5 minutes).
One at the time add each egg. Add the buttermilk and vanilla until blended.
Add the flour mixture and beat until combined.
Sprinle 2 tablespoons of the cinnamon/sugar mixture to the bottom of the prepared baking dish.
Using a spoon or your fingers, spread half the dough over the cinnamon/sugar. Sprinkle 3 tablespoons cinnamon/sugar over the dough.
Add the other half of the dough and spread over the entire baking dish. Sprinkle on remaining cinnamon/sugar.
Note on spreading: To carefully spread the dough mixture over the cinnamon/sugar mixture, use your fingertips or a spatula dampened with warm water.
Bake bars until a toothpick inserted in center comes out clean, 22 to 26 minutes. Cool bars in the dish.
Cover bars and store at room temperatures up to 4 days.