In a large bowl, using an electric handheld mixer, beat the softened butter and sugar for 30 seconds, starting on low speed and increasing to high speed as the mixture comes together. Add the egg, egg yolk and vanilla and beat on high speed until fluffy.

Add the flour, cornstarch, baking soda, and cream of tartar and mix on low speed just until combined. Cover the bowl of cookie dough and refrigerate for 1 hour.

A few minutes before the dough is done chilling, preheat the oven to 350℉ and line baking sheet(s) with parchment paper or silicone baking mats.

Remove the dough from the refrigerator and roll it into balls about 1 inch in diameter. Roll each ball in the extra white sugar or coloured sanding sugar to coat. Place 8 cookie dough balls on each pan, leaving about 2 inches of room between each cookie for spreading during baking.

Bake the cookies in the preheated oven for 8-11 minutes (I found 11 minutes was perfect in my oven, but every oven is different!) or until puffy and no longer wet-looking on top. Let cool for 5 minutes on the baking sheets before transferring to a wire rack to finish cooling. Bake the remaining cookies. Enjoy!