Preheat the oven to 350℉ (180℃). Line cookie sheets with parchment paper.
In a medium bowl, sift together the the flour, cornstarch, baking soda, cream of tartar and salt. Whisk together.
In a separate large bowl, beat together the butter and sugar until creamed (about 2 minutes).
Mix the egg, additional egg yolk, vanilla extract and almond extract into the butter mixture. Turn off the mixer and scrape down the bottom and sides of the bowl a few times.
Beat the flour mixture into the butter mixture - I typically do about 1/2 at a time, starting with the mixer on a low speed. When finished, squeeze the dough between your fingers. If it leaves sticky cookie dough attached, then mix in 1-2 extra tablespoons of flour. If you can't get all the flour incorporated - then you likely measured the flour incorrectly.
Form the dough into balls with about 2 tablespoons of dough each. Place the dough balls about 2 inches (5 cm) apart on the lined cookie sheets.
Bake 1 sheet at a time in the middle of the preheated oven. The cookies will need about 12-14 minutes or until the tops look set. Cool on the cookie sheet or at least 15 minutes before transferring to a wire cooling rack.
For perfectly circular cookies, take a large round cookie cutter (it should be bigger than the cookies). Place around each warm cookie (while still on the cookie sheet) and trace a circular motion with the cutter to mold the edges into a perfect circle. Do not cut the cookie into a circle - gently press the edges to mold into a perfect circle.