In your stand mixer (or with a hand mixer) cream sugar and butter until fluffy.
Add each egg, one at a time, beating after each egg until well combined.
Add sour cream and vanilla and beat well.
In another bowl, sift together the flour, cornstarch, baking soda, baking powder, cream of tartar and salt.
Add dry ingredients to the wet ingredients in your mixer, and mix on LOW until combined.
Cover with plastic wrap and refrigerate for at least 1 hour.
Preheat oven to 375°F.
After chilling, scoop out 1 tablespoon of dough and roll into a ball (dough will be sticky).
Place onto a parchment paper lined baking sheet. Use the bottom of a glass cup and spray with cooking spray, then dip in flour and press cookie dough balls down.
Bake for 8-11 minutes.
Remove from oven and let cool.