Pre-heat oven to moderately hot (190 degrees Celsius, fan-forced).
In a saucepan, heat water and butter until butter melts and mixture begins to boil. Remove from heat.
Stir in sifted flour (adding flour all at once) and mix in briskly using a wooden spoon. Return to heat for 30 seconds, continuously stirring. Remove from heat and stir in vanilla extract.
Transfer mixture to a bowl and leave to cool for 10 minutes.
Beat in egg yolk and then each egg, one at a time until combined. You can use a wooden spoon or slowest speed on a stand mixer.
Using a 2cm piping nozzle (ideally open star shape) and piping bag, pipe 6 x 13 cm long eclairs onto baking trays lined with baking paper. Make each end of the eclairs a little fuller, and use a knife or kitchen scissors to finish piping each eclair.
Bake for 25 minutes. Remove from oven (leaving oven on) and allow to cool for 10 minutes.
Using a bread knife, cut eclairs in half horizontally, leaving one long edge intact. Return to oven for 3 minutes, then remove and allow eclairs to cool completely on a cooling rack.