Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking pan with butter or line it with parchment paper.
Cream together the butter and brown sugar until light and fluffy, about 2-3 minutes.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the baking soda, salt, flour, and oats.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Spread half of the oatmeal cookie dough evenly in the prepared baking pan.
Spread the marshmallow creme over the first layer of dough, then sprinkle chocolate chips on top.
Dollop the remaining oatmeal cookie dough on top of the marshmallow and chocolate layers.
Bake for 25-30 minutes, or until the edges are golden brown and the center looks set.
Cool in the pan for 15-20 minutes, then transfer to a wire rack to cool completely before slicing.