Special equipment: a kitchen torch
Combine the cream and milk in a large, heavy saucier or saucepan and bring just to a simmer.
Meanwhile, add the sugar and cornstarch to the bowl of a stand mixer fitted with the whisk attachment, and mix together. Add the egg yolks, vanilla and salt to the mixing bowl just as the cream mixture is coming to a simmer. Beat on medium speed to combine, then slowly pour the hot cream down the sides of the mixing bowl to temper the sugar-egg mixture. Continue beating until smooth.
Transfer the mixture back to the saucepan and heat over medium-low heat, whisking constantly, until it thickens to the consistency of mayonnaise and is beginning to bubble.
Remove the saucepan from the heat and immediately stir in the chocolate and butter; continue stirring until melted and fully incorporated.
Immediately divide the pudding among serving cups or pour into one large bowl. Cover with plastic wrap, pressed direct onto the pudding to keep a skin from forming. Refrigerate until set and completely cool, about 2 hours.
When the pudding is ready to serve, place the graham crackers in a zip-close bag and crush into rough pieces. Place some marshmallow cream in a piping bag and pipe a mound onto each pudding. Lightly brown the top of the marshmallow with a kitchen torch. Sprinkle with graham crackers and serve immediately.
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.