Place the butter and water in a medium-sized saucepan and heat gently, stirring occasionally, until the butter melts, then allow the mixture to come to a rolling boil.
Meanwhile, sift the flour and salt into a large bowl. Make a well in the middle and add the egg.
Pour the hot liquid into the flour and stir with a wooden spoon to mix.
Spread the mixture out on a large plate with the wooden spoon and allow to cool (about 15 minutes), then wrap in cling film and place in the fridge for 30 minutes until firm. Use according to the recipe you are making.