Add cocoa, ¼ cup sugar, cornstarch and salt to a saucepan and stir together. Add in milk and stir until blended. Cook mixture over medium high heat for 10 minutes, stirring constantly.
Take ½ cup of the hot milk mixture and add it to the eggs in a bowl. Whisk together and then return to saucepan. Continue to cook over medium heat for 10 minutes.
Remove from heat and stir in vanilla extract. Cover with plastic wrap and place in the fridge for 20 minutes to cool off.
Stir in ¼ cup sugar. Spoon into dishes and top with marshmallows and walnuts.