Preheat the oven to 350 degrees F. Place the walnuts on a rimmed baking sheet. Bake until golden and fragrant, 10 to 15 minutes. Let cool slightly then roughly chop and set aside. Line the baking sheet with parchment and set aside.
While the walnuts bake, bring 1 inch of water to a low simmer in a small saucepan. Place a medium heatproof bowl over the pan, making sure the bottom of the bowl does not touch the water. Place the chocolate in the bowl and stir until completely melted, making sure the chocolate doesn’t burn, 3 to 5 minutes.
Remove the bowl from the heat and let the chocolate cool for about 5 minutes. Stir in the chopped walnuts, marshmallows and some of the graham crackers. Pour onto the prepared baking sheet and smooth into an even layer with a rubber spatula. Level the chocolate with an offset spatula, if desired. Sprinkle evenly with the remaining crushed graham crackers and some salt.
Let the chocolate bark cool completely at room temperature, about 2 hours (or refrigerate for 20 minutes or freeze for 5 to 10 minutes), then break into large pieces. Serve or store in an airtight container for up to 4 days or refrigerate for up to 10 days.