Heat 4 cups of the milk in a large saucepan over medium heat until steaming.
Whisk the sugar, cornflour and 1/2 teaspoon salt in a large bowl. Whisk in the egg yolks and the remaining 1/2 cup milk.
Whisk half of the hot milk into the egg mixture until smooth, then gradually whisk the egg-milk mixture into the saucepan. Cook over medium heat, whisking constantly, until the mixture boils. Continue to cook, whisking constantly, until it has thickened to a custard-like consistency, 3 to 4 minutes. Remove from the heat and stir in the vanilla extract.
Cool slightly, stirring a few times to prevent a skin from forming. Place a piece of plastic wrap directly on top of the pudding and refrigerate until completely cooled and thickened, about 4 hours.
Before serving, whisk the pudding vigorously until smooth and creamy. Gently fold in the marshmallows, walnuts and chocolate chips. Spoon the pudding into six to eight individual cups and top with more chocolate chips.
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