Preheat the oven to 400°F. Line a 9”x9” metal baking pan with parchment paper, and spritz with nonstick baking spray. Set aside.
In a medium bowl, stir together the flour, cream of tartar, baking soda and kosher salt. (These are your dry ingredients.) Set aside.
In another bowl, cream together the softened butter, ¾ cup granulated sugar and the egg with a hand mixer (or a stand mixer) until light and fluffy.
Sprinkle the dry ingredients into the wet ingredients, mixing until just combined.
Using your hands, press the cookie batter into the prepared baking pan.
Once the batter is evenly spread throughout the pan, sprinkle the mixture of cinnamon and 1 ½ tablespoons of leftover sugar on top.
Bake in the preheated oven for 18-20 minutes, or until an inserted toothpick comes out clean.
Let cool for 20-30 minutes before slicing and enjoying.
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!