Sift the flour and salt into a large bowl, and set aside
Place the water and butter in a medium-sized saucepan with high sides (not a low saute pan) set over a medium-high heat, stirring until the butter melts. Bring the mixture to a rolling boil, then immediately remove the pan from the heat. Add the flour, and salt, and beat very well with a wooden spoon, until the mixture comes together.
Reduce the heat to medium and replace the saucepan, stirring for 1 minute until the mixture starts to 'fur' or slightly stick to the base of the pan. Remove the pan from the heat, and allow to cool for 1 minute.
Pour about a quarter of the beaten egg into the pan and, using the wooden spoon, beat very well. Add a little more egg, and beat well again until the mixture comes back together. Continue to add the egg, beating vigorously all the time, until the mixture has softened, is nice and shiny and has a dropping consistency. You may not need to add all the egg, or you may need a little extra. If the mixture is too stiff (not enough egg), then the choux pastries will be too heavy, but if the mixture is too wet (too much egg), they will not hold their shape when spooned onto greaseproof paper.
Although the pastry is best used right away, it can be placed in a bowl, covered, and chilled for up to 12 hours, until ready to use.