In a small mixing bowl, whisk together flour, cream of tartar, baking soda, and salt. Set aside.
In a large mixing bowl (or a stand mixer fitted with the paddle attachment), beat butter and sugar together until light and fluffly.
Mix in egg and vanilla bean paste until full combined.
Mix in dry ingredients a little bit at a time until just combined into a thick dough.
Using your hands, roll balls of dough about 1.5 tablespoons each.
Roll the dough balls in vanilla sugar and place on a parchment-topped cookie sheet, leaving about 1.5" between each dough ball.
Bake for 10 minutes or until the cookies have poofed up - they will still be very soft.
While they are still warm, press down the center of each cookie with the back of a spoon.
Cool on a cooling rack.
Video
Notes
If you don't way to buy vanilla bean paste, you can substitute 1:1 with vanilla extract and add the seeds from one vanilla bean pod.You can make the dough balls and freeze them for months. Return to room temperature before rolling in sugar and baking.The cookies last for five days at room temperature. You can also freeze them for months!