1. Preheat oven to 350 degrees Fahrenheit.
2. Whisk flour, salt and baking soda in a bowl and set aside. Heat a medium saucepan over low heat. Add chocolate, butter and oil and cook until melted and smooth, stirring frequently. Remove from heat and let cool slightly, then add eggs, brown sugar, sugar, and vanilla. Stir until smooth and incorporated. Add flour and stir until just combined.
3. Grease a 10-inch cast-iron skillet with a little melted butter. Pour batter into skillet and spread evenly. Gently press cookie dough pieces into brownie batter, leaving space between cookies.
4. Bake for 30 minutes, or until sides are set and lightly toasted but center seems moist. A tester should have moist crumbs but not wet batter. Cool slightly so the cast iron can be handled. Scoop and serve with ice cream dolloped into the center.