For the rough puff pastry, place the flour in a mound onto a clean work surface and make a deep well in the centre using your fingers.
Place the butter cubes and salt into the well and, using the fingertips of one hand, work the ingredients together, gradually drawing more flour into the well with the other hand and working that in until the mixture is loosely but well combined and still with lumps of butter visible.
Gradually add the iced water and mix until fully incorporated in the mixture. (You may not need all of the water. Take care not to overwork the mixture.)
Roll the dough into a ball, wrap it in cling film and chill it for 20 minutes.
When the dough has chilled, roll it out onto a lightly floured work surface into a 40cm x 20cm/16in x 8in rectangle. Fold the rectangle into three and give it a quarter turn.
Repeat the process, giving the folded pastry another quarter turn.
Wrap the pastry in cling film and chill for a further 30 minutes.
When the dough has chilled, roll it out, as before, onto a lightly floured work surface into a 40cm x 20cm/16in x 8in rectangle. Fold the rectangle into three and give it a quarter turn.
Repeat the process, as before, giving the folded pastry another quarter turn (making a total of four turns).
Wrap the pastry in cling film and refrigerate for at least 30 minutes.
For the crème pâtissière, in a bowl, whisk the egg yolks and 20g/¾oz of the caster sugar together to a ribbon consistency. You may want to use an electric whisk to do this.
Gradually whisk in the flour until smooth and well combined.
In a saucepan, bring the milk, 40g/1½oz of the sugar and the vanilla pod to the boil. As soon as the mixture starts to boil, gradually pour the hot milk mixture onto the egg yolk mixture, stirring continuously.
Return the mixture to the pan and bring to the boil again, whisking continuously. Simmer for 1-2 minutes, then remove from the heat and transfer the mixture to a bowl.
Dust the crème pâtissière with a layer of icing sugar to prevent a skin from forming as it cools. (Once cool, the mixture can be kept in the fridge for up to three days. Remove the vanilla pod before using.)
Preheat the oven to 180C/350F/Gas 4.
Roll the rough puff pastry out onto a lightly floured surface to a 2mm/eighth of an inch thickness. Cut out six discs using a 12cm/5in pastry cutter.
Line a baking sheet with baking parchment and brush with a little cold water. Transfer each of the pastry discs onto it using a palette knife. Chill the pastry in the fridge for 20 minutes. Prick the pastry discs several times with a fork.
Spread equal amounts of the crème pâtissière mixture evenly onto each of the pastry discs, leaving a narrow margin free at the edge of each pastry disc.
Arrange the apple slices in a fan on top of the crème pâtissière mixture, starting in the centre of each disc and working outwards to the edges.
Transfer the tartlets to the oven and bake for 15-18 minutes, or until the pastry has risen and is pale golden-brown. Sprinkle generously with the remaining 60g/2½oz of caster sugar, then return to the oven and cook for a further 4-5 minutes, or until the sugar has caramelised.
Remove the tartlets from the oven and immediately transfer them to a wire rack, using a palette knife. Set aside to cool.
To serve, place one of the cooled tartlets onto each of six serving plates and spoon over equal quantities of the passion fruit pulp.