Whisk the egg yolks with the sugar, until they are pale yellow.
Pour, while whisking, the milk into the egg yolk mixture.
When all the milk is added, pour everything back in the small pan.
Bring to a boil, until the custard binds. You can check this by taking a wooden spoon and dip it into the custard. Take your finger and make a stripe in the custard. When the stripe stays visible on the spoon, the custard is done.
Sieve the custard through a fine sieve.
Whisk the chocolate through the custard and let it stand for a minute.
Then whisk again until the chocolate is incorporated.
This way you get a chocolate cream.
Let it cool down, at room temperature for about 10 minutes.
Beat the heavy cream until it gets the thickness of yogurt.
Then take a spatula and with care work it gently through the chocolate cream.
Scoop the chocolate mousse in the serving glasses and place them in the refrigerator for at least 1 hour.