Preheat oven to 350 degrees Fahrenheit. Lightly grease a 10-inch cast iron skillet with butter and set aside.
In a large mixing bowl, add your melted butter, granulated sugar, brown sugar, and vanilla, stirring to combine. Add eggs and stir until completely incorporated.
In a separate bowl, sift together your dry ingredients - flour, cocoa, and salt. Slowly stir dry ingredients into wet ingredients until combined. Scrape down the sides of your bowl.
Stir in chocolate chips and walnuts. Pour into your prepared cast iron pan and use a spatula to spread evenly to the edges. Batter will be thick.
Bake for about 28-32 minutes, or until brownies are set in the middle. Insert a toothpick into the center of the pan. If toothpick comes out with wet batter, the brownies are not done. Continue checking every 2 minutes. If toothpick comes out clean or with a few crumbs, the brownies are done.
Allow brownies to cool before slicing and serving.
Brownies will keep for 4-5 days in a sealed container at room temperature.
Dutch cocoa can be substituted with natural cocoa powder.
Walnuts can be omitted or substituted with chopped pecans, almonds, or peanuts.
Recipe can also be made in an 8x8 or 9x9 square pan - baking times will vary.
Brownies slice better at room temperature. Slicing warm brownies may cause them to crumble slightly at the edges.