Prepare the mascarpone whipped cream by beating the mascarpone, 1 1/4 cup whipped cream and the confectioners sugar in a bowl just until stiff peaks form. It only takes a minute or two.
Prepare the ganache by heating the remaining cup of cream until it’s piping hot but not boiling, then pouring it over the chocolate in a heat-proof bowl. Let stand one minute, then stir until the chocolate is melted and smooth.
Now it’s time to build those layers! Line a loaf pan with plastic wrap, leaving enough excess on the sides that it can be folded over the top. Line the bottom with a single layer of graham crackers. You can break up or crumble graham crackers to fill gaps in any of the graham cracker layers and it will all look consistent in the end.
Next, spread a layer of whipped cream. Make sure the graham crackers are completely covered. Beyond that, you can make the layers as thick or thin as you like!
Time for your first ganache layer! Spread it evenly to the corners of the pan. Repeat the layers until the pan is full, ending with a graham cracker layer. That way when you invert the cake, it will be standing on a “cake” layer. Fold the plastic wrap over the top layer and refrigerate overnight.
When the cake is nice and chilled, it’s time to flip it. Fold back the plastic wrap, lay a plate across the cake then quickly invert it. Lift the pan away and remove the plastic wrap.
The plastic leaves creases in the cake that I don’t really like the look of, so I spooned leftover ganache over the top to hide them.
Lastly, add the marshmallows and toast them, moving the torch quickly. Slice, serve and watch your cake vanish almost instantly.