Preheat your oven to 160°C.
Pour the milk, cream into a saucepan as well as the coffee and the chocolate squares. Heat over low heat and stir regularly.
Meanwhile, whisk the yolks and sugar until the mixture becomes light and fluffy.
Then pour the hot milk over the egg-sugar mixture. Mix well with a whisk at the same time to avoid cooking the eggs and making them set.
Pour this mixture into ramekins. Place them in a gratin dish and pour water halfway up the ramekins.
Bake for 35 to 40 minutes.
Let them cool, cover them then refrigerate them for at least 10 hours.
