Roughly chop the chocolate coating. Heat milk and butter in a small pot until just before boiling. Remove from the stove. Melt the couverture in it. Stir in the liqueur bit by bit. Chill for about 3 hours.
Whip the truffle mixture in 2 portions for about 1 minute each with the hand mixer until creamy. Pour into a piping bag with a medium-sized star-shaped spout and squirt one tuff into each half shell. Cool for at least 1 hour. Dust with cocoa if necessary. Store in a cool place