Line an 8x8-inch with parchment paper, leaving an overhang for easy removal. This step is essential for removing the fudge cleanly. Coarsely chop the graham crackers. Cut the butter into 1-tablespoon pieces.
In a thick-bottomed nonstick pot, add butter, chocolate chips, and sweetened condensed milk. Set the heat to the lowest setting. Stir constantly with a heat-resistant spatula, scraping the sides and bottom often. Don’t turn up the heat; slow melting prevents burning.
Once the butter and chocolate are mostly melted, remove from heat. Stir until completely smooth. Stir in salt and vanilla. Add the chopped graham crackers and marshmallows. Gently fold everything together. Some marshmallows may melt slightly—that’s okay.
Transfer the mixture to the lined pan. Lightly spray a spatula with cooking spray and press the fudge into an even layer, smoothing the top. If desired, sprinkle some more crushed grahams on top and gently press into the top. Cover and refrigerate for 4 to 8 hours, or until firm.
Lift the fudge out using the parchment overhang. Warm a sharp knife under hot water, wipe dry, and make quick, firm cuts for clean squares. This fudge is rich, so cut into small pieces.
Notes
Note 1: Milk chocolate makes the richest S'mores Fudge. For a less sweet version, use semi-sweet or dark chocolate, or a mix of the two. Ghirardelli or Guittard chips give the best flavor.Storage: Store the fudge in the fridge and enjoy it chilled. If it isn’t covered tightly or sits out too long, it will dry out and get crumbly. Because of the marshmallows, it tastes best on the first or second day.