Preheat the oven to 350ºF.
Next, place all of the egg whites into a large metal bowl and whisk the egg whites together on high until stiff peaks form -- about 3-5 minutes. Set egg whites aside.
Heat a 12-inch cast iron pan over medium heat. Add the bittersweet chocolate and butter into the cast iron.
Stir the two ingredients together until glossy and smooth.
Remove the skillet from the heat and let it sit for 2-3 minutes. Then, add the egg yolks, brown sugar, and chocolate chips to the skillet pan. Whisk the egg yolks, sugar, and chocolate chips into the melted chocolate until combined and creamy.
Finally, add the fluffy egg whites into the skillet and gently fold the egg whites into the chocolate mixture until completely combined. This may take time.
Bake at 350ºF for 15-20 minutes.
Remove the skillet from the oven and serve hot with 2-3 scoops of ice cream (we highly recommend peanut butter Ice cream).