In a small bowl, combine together the granulated sugar and cinnamon. Set aside.
In the bowl of a stand mixer, mix together the melted butter, brown sugar, and granulated sugar. Mix in egg and vanilla extract.
In a medium sized bowl, whisk together flour, cream of tartar, baking soda, and salt. Slowly mix the flour mixture into the cookie dough. Combine until just mixed.
Cover the bowl and place it into the refrigerator for 30 minutes.
Once 30 minutes has elapsed, preheat the oven to 350°F. Use cooking spray or butter an 8x8 inch baking pan. Create a sling out of parchment paper and press it into the baking pan. Spray or butter the parchment sling. Pull the bowl out of the refrigerator and divide the dough in half. Press one half of the dough into prepared baking pan. Sprinkle on the granulated sugar and cinnamon mixture. Drop dough by the spoonful over the top of the sugar and cinnamon layer. Press into place with the back of a spoon (buttered if needed), buttered fingers, or use wax paper that has been sprayed with non-stick cooking spray. It is okay if the cinnamon and sugar gets mixed in or smeared.
Cook for 22-24 minutes. While bars are cooking, start the frosting.
In the bowl of a stand mixer, add softened butter, cream cheese, and vanilla extract. Mix until fluffy.Add in cinnamon and powdered sugar, mixing well.
Pull snickerdoodle bars out of the oven. Once the bars have cooled completely, spread on the frosting.