Prepare 9x9 pan w/ cooking spray and then line pan with a sheet of wax paper (or parchment paper).

Prepare a double boiler by bringing a saucepan of water to a simmer and placing a tight fitting glass bowl on top.

Place butter, chocolate chips and sweetened condensed milk in bowl and heat until everything is melted, stirring almost constantly until well incorporated (about 5 min)

Turn off heat. Stir in vanilla and salt. Stir in 1 C mini marshmallows and 1 C chopped graham crackers.

Spread into prepared pan and sprinkle remaining marshmallows and graham crackers on top. Refrigerate 2 hours before cutting.