Lightly grease a 6.5 inch skillet with a little butter. Preheat the oven to 160°C Fan/180°C/350°F.
In the bowl of stand mixer beat butter and sugars until light and fluffy, about 5 minutes. Add the egg yolk, mixing well until combined. Scrap sides as needed. Add vanilla extract.
Sift together corn flour, salt and baking soda into mixture. Mix the flour mixture into the butter on low speed until combined.
Fold in chocolate chunks.
Divide the cookie dough into two. Place half of the cookie dough into your prepared skillet and press down to form one even layer.
Melt the Nutella in a microwave for 30 seconds until it has a thinner consistency. Then, spread out the Nutella on top using a small spatula or the back of a spoon ensuring you leave a small gap around the edges.
Grab small pieces of your remaining cookie dough, flatten them in your hand, and place them on top of the Nutella. Ensure all the cookie dough pieces join together and the layer of Nutella is completely covered.
Bake the cookie for 25 minutes, rotating the skillet half way through baking.
Remove from the oven and leave to cool for 10 minutes before diving in.