Preheat the oven to 200C/180C Fan/Gas 6 and line a baking sheet with baking paper, and line a tray with baking paper.
Put 250ml/9floz water into a pan with the salt, sugar and butter and bring to the boil. Add the flour in one go and cook for a few minutes, beating all the time, until the mixture comes away from the sides of the pan cleanly and is smooth. It will sound like bacon frying in a pan when it’s ready!
Tip out onto the lined tray and leave to cool in the fridge for 5 minutes.
Transfer to a kitchen mixer or large bowl and beat in the eggs, one at a time, then continue to beat until the mixture is rich and smooth.
Spoon the mixture into a piping bag fitted with a large, plain nozzle, then pipe 8 to 12 strips, to make long éclair shapes, onto the prepared baking sheet. Smooth out any bumps with the tip of a wet finger then sprinkle them with a little water.
Bake in the oven for 25-30 minutes until golden-brown and crisp.
Remove from the oven and transfer the éclairs from the baking tray to a wire rack to cool.
Meanwhile, put the chocolate in a bowl set over a pan of water and simmer until the chocolate has melted.
Remove from the heat and add the icing sugar and cocoa and beat together. Add 2 tablespoons of hot water from the saucepan and beat until smooth then add 3-4 more tablespoons, a little at a time, until you have a smooth shiny pourable icing. Place back over the pan of water, off the heat, to keep warm until ready to use.
Whip the double cream until soft peaks form when the whisk is removed and spoon into a piping bag.
Using the end of a clean, ink free biro, make a hole on the top of the éclair at either end. Carefully insert the nozzle into the holes, and fill with the cream.
Dip the éclairs into the fondant chocolate to cover the top evenly and leave to set before serving.