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. Cover and chill for at least 3 hours.
Remove from refrigerator and preheat oven to 325º. Let cookies sit at room temperature for 15 minutes unless you have chilled overnight, if so, then let them sit for about 30-40 minutes.
Form dough into balls using a 1.5 tablespoon ice cream scoop. 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 for 3 minutes. Add more sugar or cream if too thick or thin. Remove a small portion and add green food coloring.
Once cookies are cooled, frost and decorate with with a design and more sprinkles.