Line a 9-inch square pan with parchment paper. Lightly grease with cooking spray. Set aside.
In a medium bowl, combine ground graham crackers, sugar, and salt. Add melted butter and fold together until mixture is moistened. Transfer mixture to prepared square pan. Press into an even layer. Keep chilled in the fridge while you melt chocolate.
Melt together chopped chocolate and sweetened condensed milk in a medium metal or glass bowl over a small pot filled with an inch of barely simmering water. Stir together until chocolate is melted. Be careful not to overheat the chocolate.
Once chocolate has melted, remove from heat and pour chocolate over graham cracker crust. Sprinkle 2 cups of small marshmallows over chocolate. Press to ensure marshmallows adhere to chocolate. Place in fridge for at least 20 minutes for chocolate fudge to set.
Remove from pan. Cut into desired portions. Use a kitchen torch to toast the marshmallows.