Chop the chocolate with a knife. Melt the chocolate in a bain marie.
Mix. Heat the milk and cream, without boiling.
When the chocolate is melted, add the hot cream in several times, mixing well with a whisk. Pass this preparation to the strainer, to remove any remaining pieces of chocolate. Let cool and place in refrigerator about 30min.
When the mixture is cold, pour into the siphon. Put a gas cartridge, shake the siphon several times down. Place the siphon horizontally in the refrigerator for at least 2 hours.
When serving, shake the siphon again and dress up in the glasses.