Preheat your oven to 350 degrees F.
In a stand mixer combine the butter, brown sugar and granulated sugar, mix well on low to medium speed.
Now add the vanilla and eggs to the mixer, whisk until combined.
In a separate medium sized bowl combine the flour, baking soda and salt, mixing with a spoon or fork until well combined.
Add the dry ingredients to the mixer slowly, mix on low to medium speed until combined.
Now add both kinds of chocolate chips, and the hazelnuts to the bowl, mix by hand.
Spray the cast iron skillet(s) with non-stick spray.
Spoon the cookie mixture into one 10 or 12 inch cast iron skillet, or two 6 inch cast iron skillets.
Place in oven and bake for about 25 to 35 minutes.
Remove the cast iron skillet(s) from the oven.
You can top with ice cream, if desired. This is best served warm.
Serve with a big old hazelnut loving smile!