Pour mandarin oranges into a sieve and let them drain, collecting the juice. Puree 1/3 of the mandarins and 2 tablespoons of the juice. Stir 1/2 sachet of cream firming agent into the sauce until it thickens slightly.
Whip the cream until stiff, adding vanillin sugar and 1/2 sachet of cream firming agent. Fold the cream into the yoghurt. Layer mandarins and yoghurt in glasses, then drizzle with sauce. Decorate with meringue and lemon balm before serving.