If using a chocolate bar, chop it into small pieces so it melts more easily.
Dissolve the cornstarch in a small amount of milk taken from the recipe.
In a saucepan, combine the milk and cocoa powder, then heat over medium low heat.
Whisk continuously to prevent lumps, then add the dissolved cornstarch while whisking. The mixture should gently simmer, not boil.
Once the mixture is smooth and begins to thicken slightly, add the chopped chocolate and stir well until fully melted.
Add the pinch of salt, cinnamon, and vanilla extract.
Continue whisking and let the hot chocolate thicken to your liking. It should be thick enough to coat the back of a spoon, yet still fluid. If it becomes too thick, add a little warm milk until you reach the desired consistency.
Remove from the heat, add the remaining 30 g (1.1 oz) chocolate, and stir until completely melted.
Pour into 2 mugs or cups and pipe whipped cream on top. Decorate with marshmallows. You can use one or the other, or both.
Finish with a light dusting of cocoa powder and enjoy immediately. Hot chocolate is best served hot, as it thickens as it cools. You can gently reheat it over very low heat, in a double boiler, or in the microwave, taking care not to scorch it.