Special equipment: a kitchen blowtorch
Make the pudding according to the package instructions using the cold milk. Set aside.
In a large bowl, whip the heavy cream to soft peaks. Set aside 1 cup for the topping, and then fold the chocolate pudding into the remaining whipped cream. Fold 1 1/2 cups of the marshmallows into this mixture.
To assemble the cake: On a square serving platter, put down 1 layer of 4 toaster pastries. Top with half of the chocolate-whipped cream filling. Add a second layer of pastries, followed by the remaining chocolate-whipped cream filling. Finish with a third layer of pastries. Top with the reserved 1 cup whipped cream and sprinkle with the remaining 1/2 cup marshmallows. Using a kitchen blowtorch, lightly toast the marshmallows.
Place the entire cake in the refrigerator and allow to set up at least overnight. Before serving, top with chocolate shavings.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.