Line a 9-by-13-inch baking dish with foil, allowing the foil to hang over the sides. Heat the milk and marshmallows in a large saucepan over low heat, stirring often, until smooth. Remove from heat. Add the chocolate and stir until melted and smooth. Refrigerate for 20 minutes.
In a large bowl, with an electric mixer on medium-high, beat the cream until stiff peaks form. Working quickly, fold the whipped cream into the chilled chocolate mixture, mixing until no traces of white remain. Spoon into the prepared dish. Tap the dish on the counter to remove air bubbles. Freeze, covered, until set, at least 3 hours.
Holding both sides of the foil, transfer the frozen block to a cutting board. Cut into 24 squares. Break the graham crackers in half. Layer the chocolate squares between the crackers to make 12 double-decker sandwiches. Place the sandwiches on a plate, cover, and return to freezer until ready to serve.
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