Preheat oven to 350 degrees. Lightly grease (1) 6.5-inch cast iron skillet, set aside.
Place the chocolate chips, unsweetened chocolate, and oil in a large microwaveable bowl. Heat on HIGH for 30 seconds, stir, and heat for an additional 30 seconds. Stir until all the chocolate bits are melted, set aside.
Beat the egg, vanilla, and sugar together in a small bowl.
Add a small amount of the melted chocolate mixture and whisk to combine.
Slowly pour the egg mixture into the chocolate mixture while whisking until thoroughly combined.
Stir in the flour, baking powder, and salt.
Mix 1/4 cup of chocolate chips with 1/2 a Tablespoon of flour.
Add to the brownie batter and stir until until combined. Pour the mixture into the prepared skillet.
Bake for 26 minutes*, do not overbake! A toothpick will not come out clean when inserted into the brownie.