For the filling, combine the sugar, cornflour, salt and water in a saucepan set over a medium heat and whisk until smooth. Add the blueberries and bring to a boil, stirring constantly for 2 - 3 minutes or until thickened.
Preheat the oven to 200°C (180 C fan).
For the pastry, place flour, freeze dried blueberry powder, sugar and salt in a food processor and pulse until the mixture resembles breadcrumbs. Add the butter and water and pulse until a dough has formed.
Scoop the pastry out onto a floured surface and bring together with your hands, taking care not to work the pastry too much. Divide the dough into two pieces, one slightly bigger than the other (the bigger one will be for the lattice top).
Roll the larger piece of pastry into a large circle, about ½ cm thick. Transfer the pastry to a 8 inch pie dish. Leave a little overhang to crimp the edges. Chill in the fridge for 1 hour and remove the smaller piece of dough. Roll it out on a sheet of floured baking paper, to just bigger than your pie dish. Use a large round piping nozzle to make holes in a pattern, then put to one side while you spoon the cooled blueberry filling into the pie pan. Place the patterned lid onto the pie then trim any excess dough around the edge, leaving 1 inch around the outside. Crimp the edges using your thumbs and forefinger.
Bake in the oven for 40 minutes.
Remove the pie from the oven and let cool for at least 30 minutes before serving with double cream or ice cream.