Preheat the oven to 350 degrees F.
Line a 9-inch square pan with parchment paper.
In a medium bowl, combine together the flour, cream of tartar, cornstarch, baking powder, and salt. Set aside.
In a large bowl, add the butter and sugar and use an electric hand mixer to cream for 2-3 minutes until light & fluffy. Add the oil, egg, egg yolk, and vanilla and mix until fully combined.
Lastly, add the dry ingredients and mix on low speed until just combined.
Transfer the dough to the prepared pan and use plastic wrap to press the dough out into an even layer. Place in the fridge to chill for 30 minutes.
Make the topping by combining the sugar and cinnamon in a small bowl.
Remove the plastic wrap from the dough, and sprinkle the cinnamon sugar on top in an even layer.
Bake the cookie bars for about 25 minutes or until the edges are evenly golden.
Allow the bars to cool completely before cutting into squares..