Icebox cakes are delicious in every flavor and shape, but this s'mores version (inspired by the blog Completely Delicious) is perfect for summer. No oven required, and you get the nostalgic toasted flavor of s'mores in cake form. —Posie (Harwood) Brien
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.