In a large bowl, using wither a hand held mixer or a stand mixer with a paddle attachment, cream the butter on medium speed for a minute or so, until it's smooth. Then add the sugar and beat until it's fluffy and light in color. Beat in the egg, egg yolk and vanilla, scraping down the sides of the bowl as needed. Set aside.

In a medium bowl, whisk the flour, baking soda, cream of tartar, cornstarch and salt together until combined. With your mixer on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts. The dough will be thick and very crumbly at first, but if you keep mixing, the dough will come together. Once it does, beat in the sprinkles on low speed until they are evenly disbursed.

Cover the dough tightly and chill in the fridge for at least 3 hours and for up to 3 days. Chilling the dough is mandatory, otherwise the cookies will not hold their shape once baked.

Once dough has chilled, remove it from the fridge and let it sit out at room temperature for around 15 minutes. This will make it easier to handle, while still having a firm texture from the chilling process.

Preheat the oven to 325°F and line a couple of large baking sheets with parchment paper. Roll the dough into balls, about 1 1/2 tbsp. of dough per ball, and place them on the cookie sheets with space between them.

Bake for 9-12 minutes, until the cookie edges are just barely beginning to brown. They will still appear very soft in the centers, which is how you want them, as they will continue to firm up as they cool. If you cook them any longer, they will be hard and crunchy once they cool. Allow the cookies to cool on the baking sheets for 3 minutes, then move them to a wire rack to cool completely.

Store the cookies covered at room temperature for 3-4 days. As long as they weren't overcooked, they will stay soft and chewy for days. You could also roll the dough into balls and freeze the dough balls for up to 3 months. No need to thaw them before baking, just add an additional minute or two onto the bake time.