Pre-heat your oven to 180C.
Line a 23 to 25 cm springform cake tin with baking paper and set aside.
In a large bowl, cream together softened butter, brown sugar, and granulated sugar with an electric hand mixer until well mixed and creamy.
Add in the egg and vanilla and continue to beat with the mixer to incorporate.
In a separate bowl, mix together flour, cornstarch, baking soda, and salt.
Add flour mixture to the larger bowl slowly while you continue to beat on low.
Once your dough is mixed, fold in chocolate and peanut butter chips.
Press your dough into the prepared baking tin and decorate with extra chocolate chips, if using.
Bake on the center rack of the oven for 18-22 minutes. Try baking for 20 minutes if you like a softer center. Once your cookie cake has finished baking, flip your oven to grill on HIGH and watch it like a hawk. After about a minute the top will brown to golden perfection. Turn off broiler and remove from oven with mitts.
Allow cookie cake to cool fully before icing.