Preheat oven to 350℉. Get a cast iron skillet and grease it.
In a stand mixer, add butter and sugars, mix until creamy.
1/2 cup butter, 1/2 cup brown sugar, 1/4 cup white sugar
Add egg and vanilla, mix until combined.
1 large egg, 1 1/2 teaspoons vanilla
Add flour, baking powder and salt and mix until it forms dough.
1 1/2 cups flour, 1 teaspoon baking powder, 1/2 teaspoon salt
Add chocolate chips and marshmallows and mix by hand with a rubber spatula.
1 cup bittersweet chocolate chips, 3/4 cup mini marshmallows
Grease 10 inch skillet and press dough in it. Bake for 20 minutes.
Place in oven and bake for 20 minutes.
Check with cake tester and if it comes out clean, it is done. If it doesn't, give it a minute or two longer.
Eat, smile, and enjoy.