Gooey Marshmallow Brownie Skillet

1 Preheat oven to 350°F. Lightly grease an 8-inch cast iron skillet.
2 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.
3 Whisk in the chocolate chips until completely melted. Whisk in both sugars.
4 Add eggs, egg yolk, and vanilla. Whisk until smooth and glossy.
5 Fold in flour, cocoa powder, and salt until just combined.
6 Fold in chopped chocolate.
7 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.
8 Carefully spread the remaining brownie batter over the marshmallow. Don't worry if some fluff peeks through.
9 Bake 24–30 minutes. The edges should be set while the center still has a slight jiggle.
10 Immediately after removing from the oven, dollop additional marshmallow fluff over the top.
11 Serve warm with vanilla ice cream.