Preheat oven to 350°F. Line a 9x9” square pan with foil and spray well with cooking spray. You can use parchment in place of foil, if you prefer.
Cream butter and sugar in a mixer fitted with the paddle attachment. Cream until smooth, just a minute or so. Beat in egg, vanilla, baking soda, and salt. Mix in flour then slowly mix in oats.
Press half the mixture into the bottom of the prepared pan. It will be a thin layer. The dough is sticky so I suggest spraying your hands with cooking spray to help the dough not stick to you.
Take the lid and foil wrap off the jar of marshmallow creme. Microwave it for just a few seconds (10-20 seconds) watching it carefully - take it out just as it starts growing up out of the jar. Pour over the bottom layer in the pan and spread evenly, being careful to leave a border around the edge with no marshmallow. Sprinkle with chocolate chips.
Use two spoons to scoop the remaining dough. Flatten scoops with your hands and layer over the top of the bars, trying to be sure and cover all the edges with dough.
Bake for about 23-28 minutes, until the top of the bars are golden brown. The pan will still be slightly jiggly. Let them cool completely before attempting to slice. They’ll be gooey! Store in an airtight container for up to 4 days.