Start by making the chocolate ganache. (You can make all of it in one bowl or use three separate bowls, one for each layer of the cake, with 1 cup of the chocolate chips and 1/2 cup of the whipping cream in each bowl.)
Heat the heavy whipping cream until it starts to boil.
Pour the cream over the chocolate chips and allow them to sit for three minutes.
Stir until smooth. Place the ganache in the fridge, for 30 minutes to an hour, to allow it to cool and thicken, stirring occasionally.
Beat the instant vanilla pudding mix and milk for two minutes. Let it thicken for 5 minutes.
Beat in the marshmallow cream and whipped cream.
Line a 9x13 inch baking pan with graham crackers. Cut or break the crackers as needed.
Spread about 2 1/2 cups of the pudding mixture over the graham crackers.
Pour 1/3 of the cooled chocolate ganache over the pudding and gently spread it out.
Top with another layer of graham crackers, the second half of the pudding mixture and another 1/3 of the cooled chocolate ganache.
Top with a final layer of graham crackers and pour the remaining chocolate ganache over the crackers.
Add the mini marshmallows and toast them under the broiler or with a kitchen torch.