Pre-heat the oven to 350°F.
In a large bowl, using an electric mixer on medium, combine the butter, brown sugar and granulated sugar; beat until smooth and creamy; about 1 minute.
Add the egg and vanilla; beat to combine.
Add the flour, baking soda and salt to the butter mixture and mix until just combined.
Mix in the chocolates and the walnut halves/pieces.
Spread the batter evenly in a 12-inch cast iron skillet or other oven-safe skillet; it will be thin.
Place in the pre-heated oven and bake until golden brown and set, but still slightly gooey in the center; 20 to 22 minutes.
Remove from the oven and let stand for at least 5 minutes; serve warm or at room temperature.
The cookie is best the day it’s made; however, leftovers will last, if well-wrapped, for up to 3 days at room temperature.
