In a bowl sift the flour and set aside.
Over medium heat, place the butter, the salt, the sugar and the water in a heavy pot and bring to a boil. Remove the butter mixture from heat and add the flour all at once. Stir until it is combined into a dough.
Put the pot back onto the heat and stir constantly until the dough comes away from the sides and forms a thick smooth ball (about 1-2 min).
Transfer the dough to a bowl and let it cool for ~ 5 min.
Once the dough is lukewarm, add the eggs one at a time and mix until the dough becomes a smooth thick paste. (Note: the pastry choux dough will separate and then come together). Continue to mix until you have a stretchy thick paste which slowly falls from a spoon.
Pipe a little pastry choux onto the four corners of the baking pan to secure the parchment paper. Lightly brush the parchment paper with some water which will help the choux puffs to rise in the oven.
Pipe 16 one-inch button sized pastry choux mounds onto the baking sheet, about 2 inches apart. Using the index finger tipped in a little water, pat down any swirly tips so that it does not burn during baking. With a pastry brush, gently brush the tops of the each mound with some lightly beaten egg.
Preheat oven to 400F/200C and bake at the center rack of the oven for 15 min. Then reduce the oven temperature to 350F/180C and continue to bake for a further 10-15 min until the choux puffs are nice and golden.
Turn off the heat and with the oven door slightly ajar, leave the puffs in there for another 5-10 minutes. This is to prevent the puffs from collapsing from any sudden drop in temperature. To test if the puffs are done, split one up and check if it is almost dry inside. If not, turn the oven off, poke a couple of holes in each puff and leave them in the oven to completely cool (and dry out) before use.