Preheat your oven to 350°F (180°C) and prepare a 9-inch cake pan or pie dish by lightly greasing it. Set aside for later use.

In a medium-sized bowl, sift together the all-purpose flour, cornstarch, baking soda, and salt. This ensures even distribution of dry ingredients. Set the mixture aside.

Using a stand mixer fitted with the paddle attachment, or a hand mixer, cream the softened butter, granulated sugar, and dark brown sugar together on medium speed for about 3 minutes, or until the mixture becomes light and fluffy. This step helps to aerate the butter, giving the cake its tender texture.

Add in the egg, egg yolk, and vanilla extract, blending just until combined. Be careful not to overmix at this stage.

Gradually add the sifted flour mixture to the wet ingredients. Mix on low speed until just incorporated. It's important not to overmix to prevent a dense cookie cake. Gently fold in the chocolate chips using a spatula, ensuring they’re evenly distributed throughout the dough.

Press the cookie dough into the prepared cake pan, spreading it evenly to the edges. Bake for 20-25 minutes, depending on your oven. The edges should be a beautiful golden brown, while the center remains soft. To prevent the edges from over-browning, cover the cake loosely with aluminum foil after 15 minutes of baking.

Once done, remove the cake from the oven and place the pan on a wire rack to cool completely. For best results, serve the cake warm or at room temperature. Enhance the experience with whipped cream or a drizzle of chocolate frosting, if desired.

While this cake is at its prime when fresh, it can be stored tightly covered for up to 2 days at room temperature or in the refrigerator. Any leftovers can be frozen for up to 2 months, ensuring you enjoy this treat even later!