Put the gelatin in a bowl and cover with water.
Mash the biscuits and mix them with the melted butter. Place 4 small rings onto a platter. Put biscuit mixture in each and press down with a spoon.
Peel the apples and cut them into thin crescents. Put them to boil for 2 min. with 6 tbsp water and remove from the stove.
Heat the mandarin juice, drain the gelatin and put it in the juice. Stir until it dissolves.
In a bowl, beat the yoghurt, add the juice with the gelatin and stir.
Dry the apple crescents and distribute them over the biscuit layers. Pour in the yoghurt mixture on top and put them in the fridge for 1 hour.
Slide a wet knife along the edges of each ring and remove the rings.