Chop the chocolate with a knife and place it in a large bowl.

Bring the milk and cream to a boil. Pour the hot liquid over the chocolate, whisking to combine, then add the butter in small pieces while the mixture is still warm.

Separate the eggs, placing the yolks into the chocolate mixture. Beat the egg whites until stiff peaks form.

Gently fold the beaten egg whites into the chocolate mixture in three additions, using a silicone spatula to keep the mixture light and airy.

Divide the mousse into small glasses or bowls and refrigerate for 4 hours to set.