In a 10 to 12" cast iron skillet heat 8 tbsp of butter over medium heat until mixture is bubbly and butter become slightly browned and gives off a nutty aroma. Swirl the pan several times to prevent burning.
Remove pan from heat and stir in the remaining 4 tbsp butter until melted. Whisk in the brown sugar, granulated sugar, vanilla, salt and eggs (one egg, one egg yolk). Whisk for 20 seconds, rest 30 seconds and repeat 3 times until mixture is smooth, thick and glossy.
Whisk in the flour and baking soda until no flour clumps appear in dough. Use a sturdy wooden spoon to fold in the chocolate chips and candy pieces. Spread dough into an even layer in skillet.
Bake for 20 to 25 minutes, center will be slightly soft still. Let cool for at least 5 to 10 minutes before topping with ice cream and ice cream toppings. If slicing, let cool at least 15 to 20 minutes. Enjoy!