Preheat the oven to 350°F. Grease then line an 8 by 8-inch pan with parchment paper leaving a two inch overhang on two sides.
In a medium bowl, whisk to combine the flour, cream of tartar, baking soda, cinnamon and salt. Set aside.
In a large bowl, beat the butter and sugar with an electric mixer on medium speed til light and fluffy, about 3 minutes.
Scrape down the sides and bottom of the bowl with a spatula. Add the egg and beat til completely incorporated. Mix in the vanilla.
Use a spatula to stir the dry ingredients in in two additions. Stir until just a few streaks of flour remain. Avoid overmixing.
Pour the batter into the prepared pan. Use a spatula or an offset spatula to spread the batter evenly. Sprinkle the cinnamon sugar (recipe below) evenly overtop.
Bake for 16-18 minutes, til a toothpick comes out clean or with a few crumbs when inserted in the center.
Place pan on a wire rack. Cool completely in the pan before slicing. Use parchment paper overhang to lift cooled bars out on a pan. Set on a cutting board to slice.
Cinnamon Sugar Topping
In a small bowl, stir to combine the cinnamon and sugar.
Notes
Avoid overmixing. Overmixing will give you tough cookie bars. Be sure to stop mixing when just a few streaks of flour remain.Cream of tartar. Cream of tartar gives snickerdoodles their tangy flavor! You can find it in the baking aisle of most grocery stores or online. Checking for doneness. Insert a toothpick into the center to check for doneness. If the toothpick comes out clean or with a few moist crumbs, remove bars from the oven.Storage: Store leftover bars in an airtight container at room temperature for up to 4 days. Freezer: Wrap baked cookie bars in plastic wrap then place in an airtight container for up to 2 months. Defrost at room temperature.