In a large heatproof bowl, pour the hot heavy cream over the semisweet chocolate chips. Let stand for 30 seconds, then whisk together for 1 minute, or until thick and smooth.
Place 1 cup (85 g) of marshmallow fluff in a medium heatproof bowl and place the remaining fluff into a separate medium heatproof bowl. Add a tablespoon of milk to the bowl with 1 cup (85 g) of fluff and the remaining 2 tablespoons milk to the bowl with the remaining fluff.
Microwave each bowl of fluff for 15 seconds.
Remove the bowls from the microwave and use a fork to stir the milk into the fluff until it is thick and smooth. Cover the bowl with less fluff and store in the fridge – it will be used later for decoration.
Line a 5x8-inch (13x20cm) loaf pan with parchment paper and place a row of graham crackers on the bottom of the pan. Pour a thin layer of the marshmallow fluff over the graham crackers and use a spoon or spatula to spread the mixture evenly. Place another row of graham crackers on top of the fluff, then pour a thin layer of the chocolate ganache over the crackers and use a spoon or spatula to spread an even layer. Repeat three more times.
Cover and chill in the fridge overnight.
Remove the cake and reserved marshmallow fluff from the fridge. Use the parchment to lift the cake out of the loaf pan. Microwave the leftover marshmallow fluff mixture for 15 seconds, then use a spatula to spread the fluff over the cake in an even layer.
Optional: use a small kitchen torch to gently toast the marshmallow topping. Sprinkle crumbled graham cracker and chocolate chunks over the top of the cake.