Place the flour, salt, and butter in a food processor. Blitz until the mixture resembles coarse crumbs, about 10 seconds. Tip into a mixing bowl. Add 4 tablespoons of water and stir with a table knife until the dough just comes together. If it seems too dry, add 1-2 tablespoons more water. (Try not to make the dough too wet, because this results in a crumbly pastry.) Lightly knead the dough into a smooth ball, wrap in plastic wrap, and refrigerate for at least 30 minutes before using.