Preheat oven to 200° F
Divide the Puff pastry into 3 equal rectangle shapes + leave some leftover for the crumbs. Bake for 10 minutes.
Remove from oven and allow to cool. Using your fingers, crumble the leftover piece to as it's being used as topping.
Place the double cream, caster sugar and vanilla extract in a large bowl. Using an electric mixer, whisk the mixture on medium speed until the mixture is stiff.
Using a spatula, place some cream on the bottom piece and spread it to the corners. Do the same thing with the middle and top piece.
Sprinkle the crushed puff pastry and then the caster sugar on top. Garnish with the chopped pistachios and rose petals.
Ideally, leave it in the fridge for 2h to chill before serving.