Place the butter and water in a medium-sized saucepan, and heat gently, stirring occasionally, until the butter melts, then bring the mixture to a rolling boil.
Meanwhile, sift the flour and salt into a large bowl. Make a well in the middle of the flour, 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 cool, about 15 minutes. Then wrap in plastic wrap, and place in the refrigerator for 30 minutes, or until firm. Use according to the recipe you are making.