If you're using chocolate bars, chop them into small pieces so that they melt quickly.
Dissolve the cornstarch and salt in a little milk taken from the recipe's total amount.
Combine milk and cocoa in a saucepan over medium-low heat.
Whisk continuously to avoid lumps, then add the dissolved cornstarch while continuing to stir. The mixture should just simmer.
As soon as the mixture is homogeneous and begins to thicken slightly, add the chocolate and mix well until it melts.
Add the pinch of salt, cinnamon and vanilla.
Allow to thicken more or less to taste, stirring constantly. The hot chocolate should be smooth and creamy, yet liquid. If you find it too thick for your taste, add a little hot milk until you reach the texture you like.
Remove from heat, add the 30g (1 oz) of chocolate, and stir well to melt everything together.
Pour into 2 mugs or cups and pipe whipped cream on top. Decorate with Marshamallows. You can use either one or the other.
Dust with a light coating of cocoa powder and enjoy immediately. It should be drank while it's still hot.
The chocolate thickens as it cools. You can reheat it gently on low heat to avoid burning, in a double boiler, or in the microwave.