Melt the butter in the microwave. Set aside and cool for 5 minutes.
In a medium bowl, stir together the melted butter with the brown and white sugars. Add the egg, vanilla, and milk. Mix together.
In a separate bowl, sift together the flour, baking soda, cornstarch, and salt.
Add the dry ingredients to the wet ingredients. Mix until just combined.
Stir in the chocolate chips.
In a greased 9in round cake tin, add the cookie dough and press into the pan.
Bake at 350 °F for 20 minutes or until a knife comes out clean. Do not over bake. Cover with foil half way through if the edges start to get too brown.