Preheat oven to 350°F. Lightly grease an 8-inch cast iron skillet.
Brown the butter over medium heat until golden brown and nutty, about 5–6 minutes. Remove from the heat and whisk in the vegetable oil. Let cool for 10 minutes.
Whisk in the chocolate chips until completely melted. Whisk in both sugars.
Add eggs, egg yolk, and vanilla. Whisk until smooth and glossy.
Fold in flour, cocoa powder, and salt until just combined.
Fold in chopped chocolate.
Spread half the brownie batter into the skillet. (Instead of spreading one solid layer of marshmallow fluff, dollop teaspoon-sized mounds of fluff all over the batter. This gives you gooey marshmallow pockets instead of one continuous layer.
Carefully spread the remaining brownie batter over the marshmallow. Don't worry if some fluff peeks through.
Bake 24–30 minutes. The edges should be set while the center still has a slight jiggle.
Immediately after removing from the oven, dollop additional marshmallow fluff over the top.
Serve warm with vanilla ice cream.
