Prepare a boxed brownie according to the instructions on the back. Bake in a deep dish pie dish and allow to cool.
Pre-heat the oven on Broil (HIGH). Place 1 1/2 cups of mini marshmallows on a baking sheet with parchment paper. Broil in the oven for 1 -2 minutes or until the tops start to turn brown. Set aside.
In a medium bowl, mix together condensed milk and vanilla extract.
In a stand mixer with a whisk attachment, beat heavy cream on high speed until stiff peaks form.
Fold whipped cream into condensed milk and vanilla mixture.
Once the whipped cream is fully incorporated with the condensed milk, stir in graham cracker crumbs, crushed graham crackers, chocolate chips, and broiled marshmallows.
In a small, microwavable bowl, melt peanut butter.
Pour 1/2 of the ice cream mixture on top of the cooled brownie. Drizzle half of the peanut butter over the ice cream. Use a knife to swirl in the peanut butter. Repeat with the remaining ice cream and peanut butter.
Place in the freezer overnight or until completely frozen.
Once the ice cream is frozen, place hot fudge in a large piping bag fitted with a large flat tip. Pipe up and down along the top of the pie to cover the ice cream.
Place marshmallow fluff in a large piping bag with a large star tip. Pipe a border around the edge of the pie and immediately place back in the freezer for at least one hour.
Allow to sit for 10 minutes before cutting and serving.