Combine chocolate in a heat-proof measuring cup and set aside for later.

Separate eggs.  Add the yolks to a pan.  Keep Egg whites for later, not required.

Add sugar, milk, and cream to egg yolks.  Whisk thoroughly, until well combined.

Place pan on the stove over medium heat. 

Stir constantly for about 5 minutes with a spoon, until very hot and thick enough to coat the back of the spoon.

Remove from the heat.

Sieve the custard into the heat-proof measuring cup containing the chocolate.

Leave for 2 minutes. 

Now whisk until chocolate has melted and custard sauce is smooth and shiny, about 2 minutes. 

Add vanilla and butter; stir until butter has melted thoroughly, about 1 minute.

Pour warm custard sauce into 4 serving glasses or pots. 

Cover and place in the refrigerator until completely chilled, at least 3 to 4 hours.