Preheat oven to 350 degrees F. Lightly oil a 9" cast iron skillet.
Melt the unsweetened chocolate and butter on stove or in microwave until almost melted. Stir until completely melted. Set aside to cool for a few minutes.
In a large mixing bowl, beat egg until light yellow, 2-3 minutes. Add sugar and beat until mixed.
Add vanilla and chocolate mixture to the egg and sugar and mix until smooth. Add the flour and baking soda and stir until just combined. Pour into cast iron skillet. Rinse bowl to use for cookie.
Bake brownie for 10 minutes then remove from oven.
In a medium bowl whisk together flour, baking soda and salt. Set aside.
In a large bowl, beat together butter and sugars until creamy. Add vanilla and eggs until incorporated.
Gradually add flour mixture to butter mixture until combined. Stir in chocolate chips.
After brownies have cooked for 10 minutes, remove from oven and add cookie dough to the top in giant tablespoon size scoops. Don't completely cover the brownie, you want some peeking out of the cookie.
Return to the oven and bake until cookie is browned on top and a toothpick inserted in the middle comes out clean. About 20-25 minutes.
King Sized Brookie https://thatrecipe.com/blog/king-size-brookie/