Preheat the oven to 350°F (180°C), or 160°C for a fan oven. Grease and line a 9x13-inch baking pan with parchment paper.
In a medium bowl, whisk together the flour, cream of tartar, baking soda, baking powder and salt; set aside.
In a large bowl, whisk together the melted butter, brown sugar, and ¾ cup of granulated sugar. Add the eggs and vanilla and whisk again.
Add the dry ingredients to the wet ingredients and mix just until no more streaks of flour remain.
Spread the mixture evenly into the prepared pan.
In a separate bowl, whisk together the remaining ¼ cup granulated sugar and ground cinnamon, then sprinkle over the the surface of the cookie bars
Bake in the preheated oven for 20-25 minutes, or until golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow to cool completely before slicing and serving.
Notes
Prepare your pan. It's best to line your baking pan with parchment paper, leaving some extra on the long sides. The extra paper acts like handles that make it so simple to remove the bars from the pan once they've cooled.
Use room temperature ingredients. When all of your ingredients are at the same temperature, they will mix together more willingly, giving you a super smooth dough and a thick and chewy texture.
Avoid overmixing. Stir in the flour only until you can't see it anymore. Overmixing can lead to dry baked goods.
Cool before slicing. For clean slices, the bars must be completely cooled. Use a long sharp knife to cut these bars into 24 pieces.
Try adding frosting! A simple cream cheese frosting would be delicious with these bars.