Preheat oven to 350 degrees F. In a medium-sized bowl, whisk together the all-purpose flour, baking soda and salt. Set aside.
In a large bowl, add the butter, dark brown sugar and white granulated sugar. Cream the butter and sugar together until it's light and fluffy, about 3 minutes. Add the egg and vanilla extract and mix until thoroughly combined.
In two batches, add the flour mixture to the butter and sugar mixture until flecks of flour are no longer visible, being careful not to over mix the batter. Fold in the dark chocolate chunks and chopped walnuts.
Pour the dough into a well-seasoned, 12-inch cast iron skillet. Transfer the skillet to the oven and bake for 20 minutes. At the 20 minute mark, turn the heat down to 300 degrees and bake for an additional 15 to 20 minutes, or until the edges are golden brown yet the center is still slightly soft. Allow the big cookie to cool completely in the skillet before cutting it into slices.
Serve cookie slices room temperature with a generous scoop of vanilla ice cream.