Quickly rub together the flour, salt and butter. Form into a ball and roll thin between greaseproof paper so that it fits a 23cm flan tin. Lay it into the tin to line the bottom and up the sides. Put it into the freezer for a few minutes.
Cover pastry with buttered aluminium foil and bake in preheated oven for 20 minutes. After the 20 minutes are up, look under the aluminium foil to see if the pastry is cooked through, and bake a little longer if necessary. Remove the flan tin from the oven and immediately and carefully remove the aluminium foil. Let cool.
Combine the milk, vanilla and sugar and bring to the boil.
Mix the cornflour with the egg yolks. Off the heat, whisk into the milk until smooth. Put back on the heat and cook gently, beating continuously until a thick custard is formed. Remove from heat and stir every few minutes to prevent a skin forming.
Spread the cream over the cooled pastry and smooth out. Place blackberries on top and gently push them into the cream slightly. Allow the tart to set for 2 hours in the fridge before slicing.
You can use 1/2 a vanilla bean instead of the vanilla extract.
This tart also works well with pretty much any other fresh berry.
You can make the cream in advance, store in the fridge and spread it over the cooled pastry case when ready.