Cut 2 puff pastry slices into 4 strips of the same size each. Place the puff pastry sheets and strips on 2 baking trays lined with baking paper and chill for 30 minutes. Brush the puff pastry strips with water and sprinkle with sugar crystals. Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes until golden brown. Let them cool down.
In the meantime wash, clean, halve and stone the plums. Mix pudding powder and sugar with 3 tablespoons of prune juice until smooth. Boil up the rest of the prune juice. Stir in pudding powder, bring to the boil and simmer for 1 minute. Add the plums and fold in carefully. Bring to the boil again and let it cool down. In the meantime, chop the pistachios coarsely. Mix cream setting and vanillin sugar. Whip the cream until stiff, while letting the cream setting agent trickle in. Fold in the pistachios.
In the meantime, chop the pistachios coarsely. Mix cream setting and vanillin sugar. Whip the cream until stiff, while letting the cream setting agent trickle in. Fold in the pistachios. Pour plum compote onto the unsweetened cake base. Spread the whipped cream evenly on top. Place 2 puff pastry strips on top and divide into 2 pieces