In a bowl, whisk together brown sugar, granulated sugar, and cooled melted butter until smooth.
Add the egg, vanilla extract, and salt. Whisk until the mixture is light and fluffy.
In a separate bowl, combine flour, baking soda, and cornflour. Mix to combine.
Gradually add the dry ingredients to the wet ingredients, folding gently until just combined.
Fold in the chocolate chips.
Cover the dough with plastic wrap, ensuring it touches the surface of the dough, and refrigerate for 20 minutes.
Line an 8-inch or 9-inch pan with parchment paper, covering both the bottom and sides.
Transfer the dough to the pan and flatten it using a spatula.
Bake in a preheated oven at 170°C (350°F) for 18-20 minutes, or until the edges are golden brown.
Let the cookie cake cool completely in the pan before removing it.