Place condensed milk, vanilla extract, cacao powder and a pinch of sea salt into a medium sauce pan.
Stir, until cacao is mixed well with condensed milk and no lumps remain.
Gradually add milk. Place the hot chocolate on the stove and heat through, stirring continuously. Do not boil. I recommend tasting your hot chocolate at this point, and make adjustments according to your taste preferences. For example, if it tastes too rich for you, add a little more milk, or add a bit of sweetener if you like things on the sweeter side.
Pour hot chocolate into the cups and top with mini-marshmallows if desired.