Preheat oven to 180°C and lightly grease a 12.5cm/5" cast iron skillet with some butter or non-stick spray.
Combine the softened butter and sugars in a small bowl and mix until thoroughly combined.
Add in the yolk and vanilla and keep stirring until integrated.
Finally add in the dry ingredients and choc chips, and stir until just combined (don't overwork the mixture or it can become tough).
Press the mixture evenly into the skillet and bake for 12-14 minutes, until the cookie is golden brown.
Enjoy!