Rinse box cake tin (25 cm long; 1 1⁄2 l capacity) with cold water, line with cling film. Soak gelatine in cold water. Mix yoghurt, 100 g sugar, vanilla sugar and orange juice.
Whip 250 g cream until stiff. Squeeze out the gelatine, dissolve at low heat. Stir in 2-3 tablespoons of yoghurt cream by the spoonful, then stir into the rest of the cream. Fold in the cream immediately.
Fill half of the yoghurt cream into the box form. Place the orange cookies vertically (4-5 rows of 4 cookies each). Add the rest of the cream and smooth it down. Put them in a cool place overnight.
Peel the orange so that the white skin is completely removed. Remove orange fillets with a sharp knife between the parting skins. Finely chop the chocolate. Turn the cake onto a plate and remove the foil.
Whip 250 g cream until stiff, while letting 1⁄2 tsp. sugar trickle in. Spread the cake with it. Decorate with chopped chocolate and orange filets.