Place a 10 inch cast iron skillet over medium-low heat and melt 1 cup butter. Add in the 2 tablespoons oil and 1 cup semi-sweet chocolate chips and stir with a wooden spoon or spatula until melted. Stir in the 1/3 cup cocoa powder until combined.
In a separate mixing bowl whisk together 3 eggs, 1/4 cup sugar, and 3/4 cup brown sugar. Mix until the sugar is dissolved and the mixture is foamy, about 2 minutes of vigorous whisking. Stir in the 1 teaspoon vanilla.
Pour the egg mixture into the chocolate and whisk or stir together until fully incorporated, making sure to scrape down the sides of the pan as you mix.
Add the 1 cups all purpose flour and 1/2 teaspoon salt to the pan and fold until just combined. Do not over mix the brownie mixture for a more fudgy brownie texture.
Add additional chocolate chips on top as desired before baking.
Bake for 20-25 minutes, or until a toothpick comes out with just a few crumbs on it. Allow to cool at least 10 minutes before serving.
Notes
Nutrition facts are just an estimate of the ingredients listed.
The brownie will be fudgier as it cools, it can be crumbly while still hot.