In a medium bowl, combine the flour, salt, corn starch and baking powder and set aside.
3 cups all purpose flour, 2 tablespoon corn starch, ¼ teaspoon kosher salt, ¼ teaspoon baking powder
In a stand mixing bowl, add the room temperature butter and sugar. Mix with the paddle attachment on low until JUST combined. Don't cream them together or the cookies will spread!
8 ounces unsalted butter, room temperature, 1 ½ cups granulated sugar
Stop the mixer and scrape down the bowl with a rubber spatula. Add in the egg and vanilla and mix on low until combined.
1 large egg, room temperature, 1 teaspoon vanilla extract or paste
Add the bowl of dry ingredients and mix on low until the dough starts to come together. When there are no more large pockets of flour, stop mixing.
Grab half of the dough from the bowl and place it on top of a piece of parchment paper. Place a second sheet of parchment paper on top and press the dough down into a flat disk. Use a rolling pin to roll the dough into ⅛ inch thick round and then transfer it to a baking pan. Place the pan in the freezer while you roll out the other half of the dough the same way.
Leave both slabs of dough in the freezer for 5-10 minutes or until the dough is cold and firm. Remove the top sheet of parchment and use a 2 ½ inch round cutter to cut out the cookies. You will have about 36 rounds from the entire batch. You can re-work and roll the dough scraps twice.
Transfer 18 of the rounds to a parchment lined baking tray and place them in the freezer.
Take a 1 inch round cutter or the bottom of a piping tip to cut out the center of the other 18 cookies. These will be the tops. Place them on a lined tray and freeze them while the oven preheats to 375 F/190 C.
Bake the cookies on a lined tray, spaced 2 inches apart for 10-12 minutes or until the tops look matte and dry. You may have to bake them in batches. Keep the raw cut cookies in the freezer when not being baked so they stay cold. Let the tray cool completely on a wire rack before removing the cookies.