Take the puff pastry out of the fridge. Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Unroll the puff pastry and place on a baking tray together with the paper. Spread 1 tablespoon of sugar on top and press down gently.
Cover with a piece of baking paper. Place a fat pan (deep baking tray) on top to weigh down. Bake puff pastry in a hot oven for about 25 minutes.
Take out the puff pastry and cut it crosswise with a sharp knife into 3 equally sized strips. Let it cool down. In the meantime, sort out the blackberries, wash them carefully if necessary and drain them. Heat the jelly, mix with the berries and leave to cool.
Cut the vanilla pod open lengthwise, scrape out the pulp with the back of a knife. Stir mascarpone, vanilla pulp, lemon juice and 75 g sugar until smooth. Whip cream until stiff, fold in.
Spread half the mascarpone cream and berries on a strip of puff pastry. Place 2. strip of dough on top. Add the rest of the cream and berries. Place the rest of the pastry strips on top and carefully press down lightly.
Keep cool until serving. Dust with icing sugar.