Short Crust Pastry: Mix the flour and salt. Process together with the butter in a food processor adding as much ice water as required a tablespoon at a time until the dough all sticks together and all the flour is collected from the sides of the container.
Cut the dough into two equal-sized pieces. Roll dough pieces between two layers greaseproof paper into circles to fit a 23cm diameter tin plus its edges. Put into the fridge for 15 minutes.
Preheat the oven to 230 degrees C.
Use dried beans to keep the dough in place and pre-bake in the preheated oven for 10 minutes. Reduce the oven temperature to 180 degrees C and bake for a further 10 minutes. Remove the pastry from the oven and allow to cool completely before filling.
Filling: With an electric mixer, mix the cream cheese, vanilla sugar and cream into a creamy mixture. Spread evenly over the pastry.
Topping: Mix a couple of tablespoons of the 125ml water with the cornflour until smooth.
Place half of the blueberries into a heavy saucepan with the remaining water, sugar and salt then bring to the boil. Stir in the cornflour and continue to boil stirring constantly until the mixture is thick and clear.
Remove from the stove. Stir in the lemon juice and butter until the butter has melted completely. Add the remaining berries then immediately and carefully pour over the pastry smoothing out if necessary. Leave for at least hour to set. Store in the fridge and remove 30 minutes before serving.