Preheat the oven to 350℉.
In a large mixing bowl, mix together the softened butter and brown sugar until light and fluffy.
Add in the egg and vanilla extract. Stir until fully incorporated.
In a separate bowl, stir together the flour, cornstarch, baking soda, baking powder, and salt.
Gently fold the dry ingredients into the wet bowl, a little at a time, until a thick but smooth dough forms.
Stir in the chocolate chips.
Prepare a 12-inch round pizza pan with nonstick cooking spray.
Add the cookie dough to the pan and press into an even layer.
Bake for 12 minutes. Remove the pan, and let cool completely.
Decorate with Fluffy Homemade Buttercream. Enjoy!