Preheat oven to 350F/180C degrees. Grease a 9-inch pie dish or cake pan. Set aside.
In a medium bowl sift together flour, cornstarch, baking soda and salt. Set aside.
In a mixer bowl fitted with the paddle attachment or with a hand mixer, beat the butter, granulated sugar, and brown sugar on medium speed for about 3 minutes until creamy and fluffy. Add egg and vanilla extract and beat until combined. Add flour mixture and mix just until combined. Do not overmix. Fold in chocolate chips.
Press cookie dough into the pan. Bake for 20-25 minutes (mine is ready in 20) until golden brown but still soft at the center, covering it loosely with aluminium foil after 15 minutes so the edges won't brown too much. Remove cake from oven and set the pan on a wire rack. Allow to cool completely. Serve warm or at room temperature, with whipped cream or chocolate frosting, if you like.
Cake is best eaten the same day it's made but will keep, covered tightly, for up to 2 days at room temperature or in the fridge. Leftovers can be frozen for up to 2 months.