Preheat the oven to 375, and lightly spray a 8x8" pan with cooking oil or baking spray.
Using a stand or a hand mixer, cream the butter and sugar until light and fluffy, about 1 minute.
Add the pumpkin puree, egg and vanilla. Beat until just combined.
Whisk the flour, pumpkin pie spice, baking soda, baking powder and salt together in a bowl. Add to the butter mixture and mix until just combined. Don't over mix!
Stir in the chocolate chips with a wooden spoon.
Spread about 3/4 of the cookie batter into the greased pan, reserving 1/4 cup.
Use a spatula to spread the marshmallow fluff on top of the cookie dough layer. It's ok if the marshmallow mixes with the cookie dough batter a little. Cover most of the cookie dough batter with the marshmallow cream and sprinkle a few more chocolate chips on top.
Spoon the remaining 1/4 cup of cookie dough on top around different parts of the marshmallow layer. You don't want to completely cover the marshmallow layer, just put chunks of the cookie dough on top.
Alternatively if using mini marshmallows and chocolate chips, stir into the batter and reserve about 1/4 cup for adding on top of the batter before baking.
Bake the bars in the oven for 25-30 minutes until a toothpick comes out mostly clean. These bars will harden as they cool and the marshmallow later will stay gooey, so remember that when you're checking with a toothpick!
Let cool in the pan before cutting (bars will be very gooey if you cut into them while warm!).
Store in a covered container and heat in the microwave for a few seconds the next day to enjoy again!