First, make the custard. Pour the milk into a saucepan. Halve the vanilla pod lengthways and scrape out the seeds with the tip of a sharp knife.
Add the seeds and empty pod to the milk and set the pan over medium-low heat until just starting to bubble.
Remove from the heat, take out the vanilla pod and set aside for 30 secs to cool slightly.
Meanwhile, whisk the egg yolks, sugar, and cornflour in a bowl until smooth. Gradually pour into the warm milk, whisking constantly.
Return to medium heat and continue to whisk for 2 mins or until thickened.
When the custard starts to bubble, turn the heat low and cook for 3 mins more, still whisking. Remove from the heat.
Put 4 tbsp boiling water in a small heatproof bowl, sprinkle over the gelatine and stir briskly to dissolve.
If it doesn’t dissolve completely, stand the bowl in a pan of boiling water and stir again until dissolved.
Stir the gelatine into the custard until combined. Pour into a large bowl, cover the surface of the custard with clingfilm and leave to cool for 1 hr.
Chill for 3 hrs or until completely cold: it will feel softly set to the touch.
Meanwhile, line 2 baking trays with nonstick baking paper. Unroll the pastry, then roll it out with a rolling pin to increase the width a little; trim to a 27 x 30cm rectangle. Cut the rectangle into 18 smaller rectangles measuring 5 x 9cm using a sharp knife and scoring first to guide you.
Transfer to the baking trays and chill for 30 mins.
Preheat the oven to gas 6, 200°C, and fan 180°C. Prick the pastry rectangles with a fork and dust with 1 tbsp icing sugar.
Bake for 10-12 mins until puffed and golden, then set aside to cool completely.
Core and thinly slice the pear and toss with the lemon juice; set aside.
Use a palette knife to gently press down the 12 fork-marked pastry rectangles to flatten a little.
Spread with the blackcurrant conserve, pressing down gently to flatten slightly (this will help you stack them).
Cover the blackcurrant conserve with a layer of cream.
Stir the chilled custard until smooth, then spoon into a large piping bag with a round 1cm nozzle.
Pipelines of custard lengthways on top of the cream, then scatter over two-thirds of the berries, pear slices, and mint.
Place one blackberry, cream, and custard-topped pastry rectangle on top of another to make six stacks.
Top each with a plain rectangle. Pipe a single custard line on top and finish with the remaining fruit and mint.