Preheat the oven to 350°F (180°C). Grease a 9-inch round cake pan and line the bottom with parchment paper.
In a large bowl, beat the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add the egg, egg yolk, and vanilla extract to the butter-sugar mixture and mix until well combined.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, mixing just until combined.
Fold in the chocolate chips evenly.
Press the dough evenly into the prepared pan.
Bake for 20 to 25 minutes, until the edges are set and lightly golden but the center remains soft.
Allow the cookie cake to cool in the pan for 15 to 20 minutes before transferring to a serving plate.
Decorate with frosting, extra chocolate chips, or sprinkles if desired, then slice and serve warm or at room temperature.