Preheat the oven to 350 degrees F. Line an 8 x 8 baking pan with parchment paper and set aside.
In a large bowl, beat together the flour, brown sugar, baking soda, cinnamon, pumpkin pie spice, salt, vanilla and butter. Beat until well combined and then stir in the oats.
The mixture should be crumbly, but also be able to pack together well. If it is too wet, add a bit more flour, and if it is too dry, add a tablespoon or more of butter.
Separate this crust mixture in two and press 1/2 of the mixture into the prepared baking pan.
Bake for 12-14 minutes and then remove.
Immediately pour over the chocolate chips and spread evenly on top of the crust.
Then press the marshmallows evenly on top of the chocolate chips (there should just be one layer of marshmallows)
Crumble the remaining crust mixture on top and bake for another 5 minutes. Even though the top may still appear a bit gooey, don't bake for longer or the marshmallows will get gross.
Remove from the oven and allow to set up for at least 1 hour (or eat them immediately, but they will be pretty gooey and messy. Which personally I love!)