In a large bowl, cream the softened butter until smooth and creamy.
Add the sugar and beat until well combined.
Add the vanilla and the egg, and beat until fluffy.
Add the salt, baking powder, and cream of tartar, and mix well.
Stir in the flour a little at a time until just combined. Do not overmix.
Cover the dough and chill in the fridge for about 2 ½ hours to keep the cookies from spreading.
Preheat the oven to 350°F.
Roll the dough into balls using a #40 cookie scoop and place on an ungreased cookie sheet.
For a thinner cookie, use the back of a spoon to gently flatten the tops.
Bake for about 8 minutes, then remove from the oven.
Add the sanding sugar with a spoon, or use a small cookie cutter placed on top of the cookie and fill it with sanding sugar.
Return to the oven and bake for another 4 to 5 minutes, or until the bottoms are lightly browned.
Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack.
Notes
Chilling the dough for about 2 ½ hours keeps the cookies from spreading and turning thin and crisp.
For fluffier cookies, leave the dough in balls before baking. For flatter cookies that are easier to decorate, flatten the tops with the back of a spoon.
If you want a pretty, even layer of sanding sugar, place a small cookie cutter on top of each partially baked cookie and fill it with sugar before returning to the oven.