Puff pastry: Place the cold water and salt into a large bowl, mix in the warm melted butter. Add the flour and mix with a pastry scraper. This is called a détrempe. Transfer to a lightly floured work surface, knead lightly until blended to form a smooth dough. Flatten slightly with a rolling pin. Wrap in cling film and refrigerate for 30 minutes. Tap the dry butter with a rolling pin to soften. Cut the butter into a square, placing the trimmings on top and flatten lightly. Dust the work surface with flour and roll out the dough. Place the butter in the centre of the dough; fold the four sides of dough up and over, enclosing it completely. Lightly press to seal the butter inside the détrempe.
1st turn: Use a rolling pin to roll into a long rectangle, sealing the butter in the détrempe. Dust with flour if needed. Fold top 1/3 over the middle section. Then, fold the bottom 1/3 over the middle to form a square; make sure all the edges are aligned (1 turn).
2nd turn: Give the dough a 45° turn to the right. Lightly press the rolling pin along the edges to seal then roll out into a rectangle. Fold top 1/3 over the middle section. Then, fold the bottom 1/3 over the middle to form a square. Press lightly on the dough with the rolling pin then place two fingerprints to indicate two turns. Wrap in cling film and refrigerate for 20 minutes. Remove from the refrigerator, remove cling wrap and place the dough in front of you with the fold on the right. Roll and turn the dough twice again, as previously described and refrigerate for another 20 minutes. Roll and fold the dough twice more for a total of 6 turns. Wrap and refrigerate for 30 minutes. Preheat the oven to 200°C.
Roll out to a thickness of 2 mm; cut into a rectangle 30 x 24 cm, dock lightly with a fork. Transfer to the baking sheet lined with parchment paper and bake for 10 minutes. When the pastry starts to rise place a second parchment paper and cake rack on top. Reduce the oven temperature to 180°C and continue baking for another 20 minutes. Remove the puff pastry from the oven, increase the temperature to 200°C. Sprinkle the pastry with icing sugar and return to the oven until the pastry is caramelized. Cut 4 x 4 cm strips the length of the rectangle; then cut each into 3 to obtain 12 rectangles 10 x 4 cm.
Vanilla Chantilly cream: Whisk the cream to soft peaks. Using the point of a knife, scrape the seeds from the vanilla pod into the cream. Set aside the pod for decoration. Add the icing sugar; continue whisking vigorously to stiff peaks. Transfer to a pastry bag fitted with a round tip.
Assembly and decoration: Pipe small balls of Chantilly cream onto 8 puff pastry rectangles without leaving any space. Place the apple jam into a pastry bag and pipe a line down the center of 4 Chantilly cream covered rectangles. On the 4 Chantilly cream and apple jam puff pastry rectangles, place a Chantilly cream only garnished puff pastry rectangle. Top with pistachio garnished puff pastry rectangle.