Line a 13x9 baking pan with foil so it extends up the sides. Spray with non stick spray.
In a bowl of stand mixer or handheld mixer, cream together the butter, brown sugar and vanilla until light and fluffly.
Beat in graham cracker crumbs, flour and salt.
Add condensed milk and mix until combined.
With a wooden spoon or spatula, fold in 1/2 a cup of the chocolate chips and 1/2 of a cup of the marshmallows. Dough will be thick and sticky.
Transfer mixture to pan and spread out evenly with a spatula.
Sprinkle top with remaining chocolate chips and marshmallows.
Cover with plastic wrap and chill in fridge until mixture firms up to a fudge consistency. About 1-2 hours.
Cut into bars and enjoy!
Store any leftovers in container in fridge.
