Place an oven rack on the upper shelf. Place the marshmallows on a baking sheet lined with foil or parchment paper and broil (preferably on the low broil setting), until the tops are golden but not charred, 10-30 seconds. Carefully flip the marshmallows and allow the other sides to brown. Alternately, you can use a brulee torch to lightly caramelize the marshmallows on each side (this is what I did).
Line a sheet pan with parchment paper. Bring a medium-sized saucepan filled with 1-2 inches of water to a simmer. Top with a heatproof bowl, and add the chocolate. Stir frequently until the chocolate is completely melted.
Fold about half of the marshmallows and graham crackers into the chocolate, stirring to coat. Pour the chocolate onto the prepared baking sheet, using an offset or regular spatula to spread into a 9x10 inch rectangle. Gently press the remaining marshmallows and graham crackers into the chocolate.
Allow the bark to come to room temperature (the refrigerator will speed up this process if you’re in a hurry).
Break the bark into smaller pieces and serve at room temperature.
These are also great with a 50/50 mix of semi-sweet and bittersweet chocolate.Adapted from Food Network