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 a handmixer or standmixer with paddle attachment, cream butter and sugar together in a large mixing bowl. Start on low speed and gradually increase to medium speed. 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, around 15 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.
Add sprinkles (optional) and briefly mix on low speed, just until incorporated.
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 22 cookies.