With an electric mixer, cream the butter and sugar until light, about 2 minutes. Add egg, egg yolk and vanilla. Scrape bowl as needed.
In a separate bowl, whisk together the flour, baking soda, cream of tartar, cornstarch and salt. Slowly add to the butter mixture on low speed and mix until combined. Gently mix in the sprinkles, just enough to disburse but not so much that they bleed into the dough.
Form dough into balls using a 1.5 tablespoon ice cream scoop. Cover and chill for at least 3 hours. In the alternative, you can refrigerate the entire ball of dough, if so, let the dough sit at room temperature for 15-30 minutes so that it is soft enough to form into balls but not as soft as it was when you made the dough.
Remove from refrigerator and preheat oven to 325º. Let cookies sit at room temperature while the oven preheats. Place the balls on a parchment covered baking sheets.
Bake for 11-12 minutes until the edges are slightly browned, the centers will appear soft. Allow to cool on pan for 3 minutes then transfer to a wire rack to cool.
Frosting: With a hand mixer, beat the butter until creamy. Add confectioners’ sugar, cream, vanilla and salt and mix on low speed until combined then increase speed and mix until smooth and creamy. Add more sugar or cream if too thick or thin.
Once cookies are cooled, frost and decorate with sprinkles.