Preheat the broiler. Make the ganache: Put the chocolate in a small bowl. Bring 1/2 cup heavy cream to a simmer in a small saucepan; pour over the chocolate and let sit 5 minutes. Stir until smooth; set aside to cool.
Butter a baking sheet, then arrange 15 marshmallows on the baking sheet and broil until toasted and bubbling, 30 seconds to 1 minute; set aside to cool. Combine 1 cup heavy cream, the cocoa powder and confectioners' sugar in a large bowl and beat with a mixer on medium speed until stiff peaks form, 1 to 2 minutes. Combine the remaining 1 cup heavy cream and the broiled marshmallows in a medium bowl; beat with the mixer on medium speed until stiff peaks form, 1 to 2 minutes. (It's OK if some small pieces of marshmallow remain in the whipped cream.)
Line a 9-by-5-inch loaf pan with plastic wrap, leaving a 4-inch overhang on the long sides. Line the bottom of the pan with a single layer of graham crackers, breaking to fit as needed. Spread with half of the marshmallow whipped cream. Drizzle with 2 tablespoons of the chocolate ganache, then top with another layer of graham crackers. Spread half of the chocolate whipped cream over the graham crackers, drizzle with 2 tablespoons ganache, then top with a layer of graham crackers. Spread the remaining marshmallow whipped cream on top, drizzle with 2 tablespoons ganache, then top with a layer of graham crackers. Spread the remaining chocolate whipped cream on top, drizzle with 2 tablespoons ganache and top with a final layer of graham crackers.
Fold the overhanging plastic wrap over the graham crackers and press gently. Refrigerate at least 6 hours or overnight. Place the remaining graham crackers (about 1 1/2) in a resealable plastic bag and crush with a rolling pin until finely ground. Set aside. Refrigerate the remaining ganache.
About 1 hour before serving, preheat the broiler. Warm the reserved ganache in the microwave until pourable. Butter a baking sheet. Cut the remaining 9 marshmallows in half (parallel to the flat sides) and arrange cut-side down on the baking sheet in a tight 3-by-6 rectangle (the marshmallows should be touching). Broil until toasted and bubbling, 30 seconds to 1 minute. Let the marshmallows and ganache cool, about 1 hour.
Invert the cake onto a platter and discard the plastic wrap. Press the reserved graham cracker crumbs onto the sides of the cake. Pour the ganache on top of the cake and spread in a thin layer, letting some drip down the sides. Remove the toasted marshmallows in one piece from the baking sheet with a large metal spatula and place on top of the cake.
Photograph by Johnny Miller