Preheat the oven to 400°F. Line a 9”x9” metal baking pan with parchment paper. Set aside.
In a medium bowl, whisk or soft together the flour, cream of tartar, baking soda and salt. Set aside.
In another bowl, cream together the butter, ¾ cup of the granulated sugar and the egg with a hand mixer (or a stand mixer) until light and fluffy.
Combine the dry ingredients with the wet ingredients, mixing until just combined.
Spoon the cookie dough into the prepared pan and press to evenly cover the bottom.
In a small bowl, stir together the cinnamon and 1 ½ tablespoons of the remaining sugar on top.
Bake for 18-20 minutes, or until an inserted toothpick comes out clean.
Let cool for 20-30 minutes before slicing.
If you've tried this recipe, come back and let us know how it was in the comments or ratings!
How to store leftovers: Store the cookie bars in an airtight container at room temperature for 2-3 days tops. How to freeze: Bake the cookie bars as instructed, and let them cool completely before slicing. Line a sheet pan with wax paper, and place the sliced bars onto it, spacing them out so they don’t touch. Freeze for 4-5 hours, or until frozen solid, and transfer to a freezer bag for up to 3 months. Let defrost in the fridge, then enjoy!