Note: you will be mixing everything directly in the skillet.
In a 12-inch cast iron skillet over medium heat, brown the butter. The butter will melt, then turn foamy, and have an amber color and nutty aroma. Make sure to stir the butter frequently while it is browning. Once butter has browned, remove from heat and stir in the brown sugar and granulated sugar until smooth. I use a wooden spoon for this part.
Let the butter and sugar mixture cool at room temperature for 15 minutes. You don’t want to add the eggs into the hot mixture because it will scramble them.
Preheat the oven to 325°F.
Once slightly cooled down, stir in the eggs and vanilla extract. Add the salt, baking soda, and flour and mix to combine. Stir in the chocolate chips, chocolate chunks and the shavings from cutting the chunks. Spread the dough into an even layer on the bottom of the skillet.
Bake for 26-32 minutes (mine baked 28 mins). The edges will be lightly golden. The center will be slightly gooey still, but don’t worry that’s how it’s supposed to be! Don’t overbake it. Remove from the oven and let cool for 5-10 minutes before cutting and serving. Or you can top with a few scoops of vanilla ice cream straight into the skillet and give everyone a spoon and enjoy!