Preheat oven to 350°F. Line a baking sheet with parchment paper.
In a medium sized mixing bowl, whisk together flour, cornstarch, baking powder, baking soda and salt. Set dry ingredients aside.
Using an electric handmixer or standmixer with paddle attachment, cream butter and sugar together in a large mixing bowl. Cream for 3 minutes on medium speed until fluffy. Use a rubber spatula to scrape down sides of bowl.
Add egg and vanilla extract. Mix briefly on medium speed just until combined (15-30 seconds).
Add the dry ingredients to the wet ingredients and mix on low speed just until combined, stopping halfway through to scrape down the sides of bowl. Don't overmix.
Scoop out a 1 Tablespoon ball of dough for each cookie and drop onto the prepared baking sheet at least 2 inches apart. The dough will be sticky.
Bake for 8-11 minutes, until edges of cookies are set and have just barely browned. Let cookies cool on the baking sheet for at least 5 minutes before removing. Cookies will continue to firm as they cool. Recipe makes around 20 cookies.