Preheat oven to 190 C/374 F and grease a 10 inch baking dish or skillet.

In a large bowl combine the dry ingredients: flour, baking soda, cornflour and salt. Set aside.

In another bowl using a wooden spoon mix the melted butter, light brown sugar and granulated sugar. Whisk until there are no brown sugar lumps remaining.

Add the egg and the egg yolk. Mix with a wooden spoon until combined. Add in the vanilla extract. Add in the dry ingredient mix, and mix until just combined.

Finally add in the chocolate chips and stir gently until combined. Divide the batter in half.

Add half of the cookie batter to the prepare baking dish/skillet and spread it so that it covers the base of the dish.

Place the Reese's Peanut Butter Cups on top of the cookie batter. Cover with remaining cookie batter. You can do this either by dropping spoonfuls over the dish, or gently spread the batter with your hands so that it covers the peanut butter cups.

Bake in the oven for about 25 minutes until the edges of the cookie are golden brown. Do not over bake. Instead I take my cookie out of the oven when it's still slightly underbaked in then middle as it further cooks when taken out of the oven. NOTE: Baking times may vary according to your oven.

Top with extra pieces of peanut butter cups and ice cream if desired.