Drain the mandarins on a sieve, collecting the juice. Put 800 ml cold water, 100 ml mandarin juice and liqueur in a pot. Stir in jelly powder. Leave to swell for 5 minutes.
Add sugar. Heat while stirring until the sugar has dissolved. Put a few pieces of tangerine in each of 4 glasses and fill the glasses up to half with the jelly. Chill for about 1 1/2 hours until the jelly has slightly solidified.
Spread the rest of the mandarins, except for four for decoration, into the glasses and fill up with the rest of the jelly. Chill for another 4 hours until the jelly is firm. Whip the cream until stiff, place in a piping bag and squirt cream tuffs onto the jelly.
Decorate with mandarin orange pieces, the pastry, chocolate flakes and lemon balm.