This is a tart to celebrate the green, sugary acidity of fresh raspberries. Vanilla (in the pastry) brings out their sweetness, as does the final flourish of fresh mint, while lemon picks up their tart tones.
Make the pastry. Beat the butter, icing sugar and vanilla seeds in a stand mixer fitted with the beater, on medium speed for 2–3 minutes, scraping down the inside of the bowl from time to time, until pale and creamy. One at a time, add the egg yolks, beating well between each addition. Add the lemon zest and mix again until combined.
Sift the flour and salt into the bowl, add the double cream and mix again briefly until the dough starts to clump together, taking care not to overwork it. Add a little more double cream, if needed, to bring the dough together. Tip the dough out onto a work surface and lightly knead it to bring it together into a neat ball. Flatten the dough into a disc, wrap it and chill it for at least 1 hour, until firm.
Begin the crème diplomat filling. While the pastry is chilling, pour the milk into a saucepan and add the vanilla seeds and pared lemon rind. Place the pan over a low heat and slowly bring the milk to the boil. Remove from the heat and leave to infuse for 20 minutes.
Soak the gelatine leaves in a bowl of cold water for 5 minutes, until soft.
Using a balloon whisk, whisk the egg yolks, caster sugar, cornflour and salt in a large mixing bowl for 1 minute, until pale.
Bring the milk back to the boil. Remove the pared lemon, then pour one quarter of the hot milk into the egg mixture, whisking continuously until thoroughly combined. Gradually add the remaining milk, whisking continuously until smooth.
Return the egg mixture to the pan, then place the pan over a low–medium heat. Cook for 2 minutes, whisking continuously, until the custard thickens to the consistency of softly whipped cream, and you can no longer taste the cornflour.
Squeeze any excess water from the gelatine leaves and add the leaves to the custard. Whisk until melted and thoroughly combined, then pass the custard through a sieve into a clean bowl. Cover the surface with baking paper to prevent a skin forming and leave the custard to cool to room temperature, then chill it for at least 2 hours, until set.
Make the tart case. Roll out the pastry on a lightly floured work surface into a neat disc about 2mm thick and large enough to line the base and sides of the tart tin. Transfer the pastry to the tin, pressing it neatly into the base and the sides. Trim the excess from the top and chill the pastry case for at least 20 minutes. Meanwhile, heat the oven to 180°C/160°C fan/350°F/Gas 4.
Prick the base of the chilled tart case all over with a fork, line it with baking paper and fill it with baking beans or rice. Bake the tart case on the middle shelf for 20 minutes, until the top edge starts to turn crisp and golden. Remove the paper and beans or rice and return the tart case to the oven for a further 5 minutes, until the base is crisp. Leave to cool.
Finish the crème diplomat filling. Whip the cream with the icing sugar until it holds soft peaks. Whisk the cold custard until smooth, then fold in half of the whipped cream to loosen. Fold in the remaining cream until combined.
Assemble and decorate the tart. Place the pastry case on a serving plate, fill it with the crème diplomat and spread it level with a palette knife. Pile the raspberries on top of the custard to generously cover the tart. Chill the tart for 20 minutes, then dust the top with icing sugar and decorate with mint leaves to serve.