In a large bowl, sift together the flour, cornstarch, baking powder, baking soda and salt. Whisk together and set aside.
In a separate large bowl, beat the butter, granulated sugar, brown sugar and lemon zest together until combined. There should be no lumps of butter or brown sugar when you're finished.
Beat the eggs, vanilla extract and lemon extract into the butter mixture. Turn off the mixer and scrape down the sides and bottom of the bowl to ensure that there are is no butter stuck.
Mix the flour mixture into the butter mixture about ½ at a time, starting with the mixer on a low speed. The dough will be thick. Turn the mixer off and squeeze the dough between your fingers. If it sticks (AKA leaves cookie dough on your fingers) beat in 1 extra tablespoon of flour.
Cover the bowl and refrigerate for at least 2 hour or up to 48 hours.
When ready to bake, preheat the oven to 375℉ (190℃). Line cookie sheets with parchment paper or baking mats.
Form the dough into balls with about ¼ cup to ⅓ cup each. I always use a scale - each of my cookies weighed 3½ ounces (100 grams). I got 14 cookies in total. Place the cookie dough balls at least 2½ inches (about 6 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 and are starting to turn golden. (Keep the remaining unbaked cookie dough balls in the fridge as the first tray bakes).
Remove the cookies from the oven. For perfectly round cookies, take a large biscuit cutter (or cup/mug) - it should be larger than the cookies. Place it around each cookie, and gently trace a circular shape to mold each cookie into a perfect circle. Note - you are not cutting the cookie into a circle, you are simply pressing the sides into a perfect circle. Cool the cookies on the cookie sheet.