This giant cookie packs a double-chocolaty punch.
Adjust oven rack to upper-middle position and heat oven to 375 degrees. In 10-inch cast-iron skillet, melt butter over medium heat. Turn off heat and use rubber spatula to scrape butter into large heatproof bowl. Set skillet aside on cool burner (don’t wipe it out or wash it).
In medium bowl, whisk together flour, cocoa, baking soda, and salt.
Add brown sugar and oil to bowl with butter and whisk until well combined. Add egg and vanilla and whisk until smooth.
Add flour mixture to butter mixture and use rubber spatula to stir until just combined and no dry flour is visible. Add chocolate chips and stir until evenly distributed.
Scrape cookie dough into now-empty skillet. Spread and push dough into even layer covering bottom of skillet.
Transfer skillet to oven and bake until edges of cookie are set and top looks dry and shiny, 16 to 18 minutes.
Use oven mitts to transfer skillet to cooling rack. Place oven mitt on skillet handle as a reminder that handle is HOT. Let cookie cool in skillet for 30 minutes.
Making one giant cookie might actually be easier than making a whole bunch of regular-size cookies. There’s no scooping or rolling dough, and melting the butter right in the skillet has the added benefit of also greasing the pan, preventing our cookie from sticking as it bakes. Cut your Skillet Brownie Cookie into slices and serve with a scoop of ice cream or a dollop of whipped cream.