Cut the butter into cubes. Sift the flour into the butter and rub together between the fingers and thumbs in a mixing bowl until you're left with an even crumb.
Add the cold water to the crumb and bring the mixture together into a ball which should then rest in the fridge for at least 30 minutes. This ensures the mixture stays workable during the next step.
Take the dough out of the fridge. Flour a rolling pin and roll out until a few millimetres thick.
Cut out some rounds and push the pastry into a glass ramekin or similarly sized oven-safe dish.
Cover with baking beads or uncooked rice and bake for 15 minutes in a 200 degree Celcuis pre-heated oven.
For the filling, whisk together an egg and sugar until runny and consisnent.
Add the unsalted butter, the juice of 1 lemon and the cornflour and mix again.
Heat until a custard forms (see tips) and remove from the heat to cool slightly.
Try replacing with blueberry