Make 3 layers of chocolate cake, following package instructions for 8-inch cake pans. Let cool completely before using a serrated knife to remove any domed tops before frosting.
For the buttercream:
In a medium bowl, beat the butter and shortening together until completely smooth, creamy and well-mixed.
Add the marshmallow cream, salt and vanilla extract. Stir to combine.
Add the powdered sugar in 3 to 4 batches, to avoid making a mess, stirring well between additions. Mix in the milk a tablespoon at a time until you get a soft, spreadable frosting.
Remove 2/3 cup of frosting and place into a small bowl. Mix the cocoa powder into this frosting then scoop into a disposable piping bag and set aside.
To the remaining frosting the medium bowl, add 6 tablespoons of the graham cracker crumbs and stir through to combine. Reserve the remaining 2 tablespoons of graham cracker crumbs for garnish.
To make the meringue:
Fill a medium pot with 2-inch depth of water and place over a medium-low heat on the stove. Set a large, heatproof bowl over the pot. Add the egg whites and sugar to the large bowl and stir constantly until the sugar has dissolved (the mixture shouldn’t feel gritty when you rub some between your fingertips) and registers 160 degrees F on a thermometer.
Remove the bowl from the pan and use electric beaters to whisk until cool, very fluffy and thickened - you should be able to lift up the beaters and form stiff peaks in the mixture which do not sink back into the meringue.
To make the ganache:
Place the chocolate chips into a medium bowl.
Heat the heavy cream in a small pot over a medium heat until steaming. Pour this over the chocolate in the bowl and let sit for 5 minutes so the chocolate can melt. Only then, stir together until smooth. Let cool for 10-20 minutes at room temperature until slightly thickened. Transfer the ganache to a disposable piping bag.
Place a dab of graham cracker buttercream onto a cake board or serving plate and place on the first cake layer to secure it to the board.
Scoop roughly 1/3 cup of graham cracker buttercream onto the layer and spread it out into an even layer over the top of the cake layer. Top with the second cake layer and spread on another ~1/3 cup of graham cracker frosting. Top with the final cake layer and spread another ~1/3 cup of frosting over the top of the cake.
Transfer the remaining graham cracker buttercream to a piping bag.
Snip the tips off the piping bags containing the chocolate and graham cracker buttercreams – you want the holes to be about 1 inch wide.
Pipe alternating horizontal rings of chocolate and graham cracker buttercream around the outer sides of the cake to create a stripy effect around the cake. Use a flat cake scraper (or a metal bench scraper or palette knife) to smooth the buttercream on the outside of the cake into an even, smooth layer - it’s helpful to use a revolving cake turntable here as you can spin the cake with one hand very easily as you hold the cake scraper straight against the edge of the cake.
Snip the tip off the piping bag containing the warm chocolate ganache. Pipe ganache around the top edge of the cake, allowing it to drip down the cake in vertical drips. Use the remaining ganache to cover the top of the cake, using an offset spatula if needed to spread into an even layer. Let set (you can chill in the fridge to speed this up) until the ganache is set.
Once the ganache has set, scoop the meringue from the bowl onto the top of the cake. Make dramatic dips, peaks and waves in the meringue. Use a kitchen blowtorch to lightly toast the meringue all over.
Press the reserved graham cracker crumbs over the bottom edge of the cake.