Whisk together flour, salt, baking soda, and cream of tartar.
Cream together butter and sugar until light and fluffy, about 2 minutes.
Add eggs, one at a time, beating well between each addition.
Add milk and vanilla extract, beat until combined.
Slowly add dry ingredients into the creamed mixture.
Mix until everything just comes together, scraping down the bowl as needed.
Combine cinnamon and sugar.
Sprinkle half in the bottom of a greased 9x13 pan. See note.
Spread cookie dough into the 9x13 pan on top of the cinnamon sugar.
Press into and even layer. Sprinkle remaining cinnamon sugar mixture evenly on top.
Bake for 25-30 minutes, or until the top is golden brown and a toothpick comes out slightly wet.
Notes
If you want thinner bars, follow the recipe as written but use a baking sheet instead of a 9x13 pan. This will give you double the bars at about half the height. The dough won't quite reach to the edges, reference this post for making cookie bars on a sheet tray.