Make the base: In a double boiler (you know the rules, don’t let that water touch the bottom of your top pot or bowl), add the milk chocolate chips, bittersweet chocolate chips, sweetened condensed milk and salt. Cook, stirring, until the mixture slightly thickens, 8 to 10 minutes. Transfer to a small baking dish and then refrigerate until solid, 2 to 3 hours.
Make the coating: Preheat the broiler. Line a large rimmed baking sheet with parchment paper and spread the mini marshmallows in a single layer on the baking sheet. Broil until the marshmallows are black—not brown but black, 2 to 4 minutes. (Rotate the baking sheet as needed.)
Allow the marshmallows to cool completely (sliding the parchment off of the sheet and onto a plate and placing it in the fridge helps). Once the marshmallows are cooled and hardened, remove and finely chop. Add them to a large bowl with the graham cracker crumbs and stir to combine.
Remove the truffle base from the refrigerator and, using a small scoop or spoon, make about 36 balls (a bit smaller than a golf ball), roll each in the marshmallow–graham cracker coating and place on a baking sheet. Refrigerate until ready to serve. Serve chilled or at room temperature.