Preheat an oven to 350 F and set an oven rack in the center of the oven. Grease a 10-11 inch cast iron skillet with butter and line the bottom of the skillet with a round of parchment paper.
Make the cookie dough. In a small bowl, whisk together the flour, baking soda, and salt and set aside. In a medium mixing bowl, combine the cooled melted butter with the brown sugar and white sugar. Whisk vigorously until the sugar and butter mixture is smooth. Whisk in the vanilla and egg yolks until smooth. Use a rubber spatula to fold in the flour mixture and chocolate chips until no streaks of flour remain. Refrigerate the dough while you make the brownie batter.
Melt the chocolate mixture. In a small saucepan, combine the cubed butter, chopped chocolate, oil, and cocoa powder. Whisk over medium-low heat until the butter and chocolate are melted and the mixture is very smooth with no clumps of cocoa powder remain. Let the mixture cool slightly.
Whisk the eggs and sugar. In a large mixing bowl, whisk the brown sugar, white sugar, vanilla, and eggs for 3-4 minutes until very foamy. You could also use a hand mixer here. Whisk in the slightly cooled chocolate mixture until everything is combined.
Fold in the dry ingredients. In a small bowl, mix the flour, salt, and baking powder and then fold that mixture with the chocolate chips into the chocolate mixture until no streaks of flour remain.
Assemble and bake. Spread the brownie batter in an even layer in the prepared skillet. Use a cookie scoop to dot the top with scoops of the cookie dough. Press the cookie dough into the brownie mixture and dot the top with more chocolate chips. Bake for 25-30 minutes, until the cookie dough is just barely golden brown. Cool completely before cutting into wedges or squares.