Preheat the oven to 180C (160C Fan) Gas 4 and line two baking trays with baking parchment. Draw six 14cm lines on the back of each sheet of parchment to act as your template.
Put the butter, salt, sugar and 120ml water in a medium pan over a medium-high heat. Once the butter has melted and the mixture is at a rolling boil, add the flour and quickly stir together with a wooden spoon until the mixture forms a dough.
With the pan still on the heat, stir vigorously for 2 minutes, then tip the dough into a bowl and beat for a few minutes until it stops steaming. These two actions help to cook the flour and dry out the dough, which in turn helps it to absorb more egg. This helps the choux pastry to expand properly as it bakes.
Add the eggs, one at a time, beating until fully absorbed before adding the next. Depending on the flour used and how much water evaporated as you made the dough, the choux pastry will need varying amounts of egg, so the above is given as a guide.
With this recipe I usually add two eggs and then very slowly start adding the remaining egg, checking the texture of the dough after each addition. You are looking for a dough that has a shine and when it is lifted from the bowl, it should fall from the spatula in a ribbon that forms a "V" shape. If the dough doesn't contain enough egg, it won't expand properly and will be prone to cracking as it bakes; if there is too much egg, the dough won't hold its shape and will collapse as it bakes.
To prevent the dough from drying out and forming a skin, immediately put the dough into a piping bag with a 1.5cm plain round piping tip.
To pipe, hold the piping bag at a 45 degree angle and with the piping tip touch the parchment pipe 12 straight lines. Brush each eclair with beaten egg, then bake for 25-30 minutes until golden brown and crisp. Turn off the eclairs and leave in the oven to cool for 30 minutes before filling and decorating.