In a large bowl using a hand-held mixer or stand mixer with paddle attachment, beat the butter, brown sugar, and white sugar together on medium speed until combined and creamy
Beat in the egg and vanilla
In a separate bowl, combine the dry ingredients: flour, cornstarch, baking soda and salt
Add into the wet ingredients, then beat on low speed until combined
Cover dough tightly with plastic wrap and chill for at least 1 hour
Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. Preheat oven to 350°F (177°C).
Use an ice cream scoop to form the dough into balls and put them on a baking sheet (I recommend using Silpat, a silicon baking sheet)
Bake for 10-12 minutes, until barely golden brown around the edges. The cookies will look soft, let it cool on the baking sheet for 5 minutes, then transfer to a cooling rack.
Let it cool completely and enjoy!