Melt chocolate in a heatproof bowl set over a pan of simmering water, stirring until smooth. Remove from heat, and let cool until slightly thickened. Holding graham cracker by the edges, dip 1 side of each cracker in chocolate, letting excess chocolate drip back into bowl. Put dry side down on a parchment-lined rimmed baking sheet. Refrigerate until set, about 30 minutes.
Press each marshmallow onto the tip of a skewer or a clean whittled stick. Roast marshmallows, turning occasionally, until toasted to desired doneness. Place 1 or 2 marshmallows on a cracker, and top with another one.