Line an 8-inch springform pan with plastic wrap, leaving a 5-inch overhang all around.
In a medium bowl, heat the chocolate in the microwave on high in 15-second increments, stirring occasionally, until melted and smooth. Add 3/4 cup cream; whisk until totally combined (the mixture will thicken).
In a large bowl using an electric mixer, beat 2 1/4 cups cream just until soft peaks form; add a few large spoonfuls to the chocolate mixture. Fold to blend. Add the remaining whipped cream; fold to blend.
Spread about two-thirds of the chocolate mixture (about 3 cups) into the prepared pan in an even layer. Spread one side of each graham cracker square with about 1 tbsp. marshmallow cream. Insert the graham crackers upright into the chocolate mixture so that the edge of the cracker is flush with the top of the pan. Starting at the outer edge of the pan, arrange the crackers about 1/4 inch apart, pointing inward toward the center of the cake, like the spokes of a wheel. Insert the remaining crackers upright in the center of the cake. Spread the remaining chocolate mixture over the top of the cake, smoothing it over the crackers and pushing to fill the spaces between them. Fold the plastic wrap overhang to cover. Refrigerate until the crackers soften, at least 8 hours.
Preheat the broiler. Peel back the plastic from the top of the cake; invert onto a plate. Remove the pan sides and bottom. Peel off the plastic wrap. Run a knife under hot water; wipe dry. Run along the top and sides of the cake to smooth any creases.
Place the marshmallows on a baking sheet; broil until toasted, 2 to 3 minutes. Place on the top of the cake, pressing to adhere. Serve immediately.