These snickerdoodle cookie bars have a soft & chewy texture, delicious butter flavor, and a crunchy layer of cinnamon sugar. These are way easier than making snickerdoodle cookies, and you don't have to chill the cookie dough!
Preheat the oven to 350F degrees. Line an 8x8 or 9x9 inch square baking pan with aluminium foil and lightly grease with butter or nonstick cooking spray, or line with parchment paper. (If lining with parchment paper, you don't need to grease).
In a large bowl stir together the melted butter and sugars until evenly combined and no sugar lumps remain. Ensure that the butter isn't still hot, then stir in the egg and vanilla until combined. Then carefully stir in the flour, cornstarch, cream of tartar and salt until no flour lumps remain. Spoon/spread the mixture into your prepared pan.
In a small bowl - or the bowl you used to make the batter - combine the sugar, cinnamon and nutmeg for the cinnamon sugar mixture. Sprinkle the mixture over your cookie bar batter until it's evenly covered and you can't see the cookie batter underneath. You may have some extra if you use an 8x8 inch pan instead of a 9x9 inch pan.
Bake in your preheated oven for 22-25 minutes or until you can start to see the sides of the bars just starting to pull away from the edges. Allow to cool, then remove from the pan and cut into 9 or 12 bars.