Preheat oven to 350°. Butter or spray a piece of parchment paper with nonstick cooking spray and place in a 9x13 baking dish.
In a large bowl, whisk together the flour, pumpkin pie spice, baking soda, and salt.
In a separate bowl, beat the butter with an electric mixer until creamy. Add the sugar and beat until fluffy. Mix in the egg, pumpkin puree, and vanilla. Slowly add in the flour mixture and mix until combined. Fold in the chocolate chips and marshmallows with a rubber spatula. Spread evenly into baking dish.
Bake 35-40 minutes, or until a toothpick inserted in the center comes out clean. Let cool 20 minutes in dish before transferring to a wire rack to cool completely. Cut & enjoy!