In a medium size bowl, whisk flour, baking soda, cream of tartar, and salt together. Set aside.
Mix butter, powdered sugar, and vanilla until creamy.
Add egg and mix well. While mixing at low speed, slowly add in flour mixture just until combined.
Using a medium cookie scoop, scoop dough into ¾ inch – 1 inch balls.
Arrange dough at least 2 inches apart onto a baking sheet lined with parchment paper (or greased baking sheets). Using a fork, flatten tops of cookies.
Sprinkle tops with colored sanding sugars.
Bake 8-10 minutes until lightly colored on edges.
Cool for 5 minutes; transfer to wire rack to completely cool.
Recipe by Sweet Dash of Sass at http://sweetdashofsass.com/2013/12/creamy-butter-cookies/