S'Mores Icebox Cake

First make the pudding. In a heavy-bottomed medium saucepan, whisk together the cornstarch, sugar, salt, and cocoa powder. Slowly whisk in the milk, cream, and vanilla and bring the mixture to a boil over medium heat. As soon as it boils, reduce to a simmer and cook, whisking constantly, until the mixture begins to thicken (about 3 to 5 minutes). Set aside to cool.
Whip 1 cup of the heavy cream with 2 tablespoons of the mascarpone and the vanilla until stiff peaks form.
Line a 9- by 5-inch loaf pan with plastic wrap. Place a layer of graham crackers at the bottom (you will need to break some of them to fit snugly). Follow with a layer of marshmallows, then a layer of mascarpone whipped cream to cover and fill in the spaces between the marshmallows (reserving 1/2 cup for the topping), then a thin layer of cooled chocolate pudding. Repeat the layers until you reach the top of the pan (you should be able to get 3 layers of each), and finish with graham crackers.
Place plastic wrap over the top of the pan and put the cake in the refrigerator overnight (or for at least 8 hours).
Remove the cake. Take off the top layer of plastic wrap and invert the cake over a serving platter. Remove the rest of the plastic wrap.
Whip the remaining cup of cream and 2 tablespoons of mascarpone. Spread this whipped cream over the top and sides of the cake. Top with the remaining marshmallows and briefly toast them using a kitchen torch.