Heat the butter and water gently in a medium-sized saucepan, stirring occasionally, until the butter melts, then allow the mixture to come to a rolling boil.
Sift the flour and salt into a large bowl. Make a well in the middle and add the egg.
Pour the hot butter liquid into the flour and stir with a wooden spoon to mix.
Spread the mixture out on a large plate with a wooden spoon and allow to cool (about 15 minutes should do), then wrap in cling film and place in the fridge for 30 minutes until firm. The pastry is now ready for whatever delicious pastry recipe you wish to make.