Preheat the oven to 350 degrees Fahrenheit.
Add the butter and sugar to a large bowl, and beat until light and fluffy.
1 cup butter, 1 cup light brown sugar
Add the oats, flour, graham cracker crumbs, baking powder, and salt. Beat again on low until the ingredients are well-combined a crumbly.
1 cup old-fashioned oats, 3/4 cup all-purpose flour, 1 cup graham cracker crumbs, 1 teaspoon baking powder, 1 teaspoon kosher salt
Press ½ of the oat mixture into the bottom of a 10-12” cast-iron skillet. Sprinkle ½ of the marshmallows and chocolate chips on top followed by the remaining oat mixture.
2 cups mini marshmallows, 1 cup semisweet chocolate chips + more for topping
Bake for 15 minutes.
Remove the skillet from the oven, and add the remaining marshmallows and extra chocolate chips, if desired.
Bake again for 5 minutes.
Remove the skillet from the oven, and cool for 10 minutes.
Serve warm, and enjoy!
Store in an airtight container at room temperature or in the fridge for 1-2 days.