Step 1

Preheat oven to 350F/180C and lightly grease an 8 or 9 inch springform pan or a cake pan with removable bottom.

Step 2

In a medium-sized bowl whisk together the flour, baking soda, salt, and cornstarch. Set aside.

Step 3

In the bowl of an electric mixer combine butter and sugars. Mix on medium for around 3 minutes, until light and fluffy.

Step 4

Stir in vanilla, egg, and egg yolk, making sure to mix until fully combined. Scrape down the sides of the bowl to ensure everything is evenly combined.

Step 5

Gently fold in dry ingredients then stir in the chocolate chips until just combined.

Step 6

Pour cookie dough into prepared pan and gently press into an even layer.

Step 7

Bake the cookie cake for 20-30 minutes depending on what size pan you use. For a thick cookie cake as pictured I used an 8-inch cake pan and baked the cake for 30-35 minutes, insert a toothpick into the center to check for any wet batter. For a thinner cake bake in a 9-inch cake pan or pie plate, and bake for 20-25 minutes until lightly golden brown and fully set. Make sure to rotate the cookie cake halfway-thru, if the cookie is browning too much then cover with foil for the remaining bake time

Step 8

Let the cookie cake cool in the cake pan on a wire rack for at least 30 minutes. Using a springform pan or cake pan with removable bottom makes it much easier to remove the cookie cake in one piece.

Step 9

Once removed from the pan let the cookie cool for at least two hours.

Step 10

Decorate cake with whipped cream, buttercream, sprinkles and/ or ice cream.  Store at room temperature in an air-tight container.