Preheat oven to 350°F/180°C. Line 3 cookie sheets with parchment paper. In a medium mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar until light and fluffy, for about 3-4 minutes. Add the egg and vanilla extract and mix again.
Use a medium cookie scoop to scoop the cookie dough onto the prepared baking sheets. Press additional nuts and white chocolate chips onto the cookie dough balls if desired. Bake in the preheated oven for 7-9 minutes.