Preheat your oven to 350°F (175°C). Grease and line an 8 inch square baking pan with parchment paper.
In a small bowl, whisk together flour, cream of tartar, baking soda and salt.
In the bowl of your stand mixer fitted with a paddle attachment or a large bowl with a hand mixer, beat together softened butter and both sugars for 2-3minutes until smooth.Make sure to scrape the sides of the bowl.
Beat in the egg, egg yolk and vanilla extract on medium speed until fully combined.
With the mixer on low speed gradually stir the dry ingredients into the wet ingredients until just combined, making sure not to over mix.
Press the dough evenly into the prepared baking pan.
In a small bowl, combine the 2 teaspoons of cinnamon and ¼ cup of sugar. Sprinkle evenly over the top of the dough.
Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the bars cool completely in the pan before slicing.