To make the pastry, put 100g of the flour in a bowl with the icing sugar and rub in the butter with your fingertips until it resembles fine breadcrumbs.
Add 2 of the egg yolks and mix to a smooth dough. Wrap the pastry in clingfilm and chill for 30 minutes.
Butter a 20cm tart tin, then roll out the pastry on a lightly floured surface until larger than the tin.
Gently press the pastry into the base and sides of the tin and trim the overhanging pastry a little above the edge of the tin to allow for shrinkage.
Prick the base all over with a fork and chill for 15 minutes.
Preheat the oven to 190°C, fan 170°C, gas mark 5. Line the pastry with baking paper and fill with baking beans.
Cook for 10 minutes, then carefully remove the paper and beans and return to the oven for 5-10 minutes more until pale golden brown around the edges and base.
To make the crème patissière, whisk the remaining 3 egg yolks and the caster sugar together for 4-5 minutes until very thick and pale.
Sift a tablespoon of plain flour and the cornflour over the surface then gently fold together.
Pour the milk into a medium saucepan, bring just to the boil then gradually whisk into the egg mixture until smooth.
Return the mixture to the pan and cook over a medium heat, stirring until very thick. Take off the heat and stir in the vanilla.
Cover the surface with a piece of crumpled and wetted baking paper to stop a skin from forming and leave to cool.
Take the tart case out of the tin and set on a plate or cake stand.
Melt the dark chocolate in a bowl set over a saucepan of barely simmering water, then spoon over the inside of the tart case. Leave for 20 minutes or until set.
Make up the jelly according to pack instructions using half the water. Cool then chill for 20 minutes in the fridge until just beginning to coat a spoon thickly.
Spoon the crème patissière into tart case. Pile the strawberries on top, carefully mounding in the centre.
Brush all over with some of the cooled jelly (you will not need all of it) and chill for 30 minutes until the jelly has fully set.
Sprinkle with white chocolate shavings to decorate and serve in thick slices.
Cook's tip: For a special occasion, you could decorate the tart with indoor sparklers for a real treat. Also, the leftover jelly can be cooled and packed for children's lunches.