Preheat the oven to 200C/400F/Gas 6.
Place 250ml/9floz water and the butter into a saucepan and bring to the boil.
Simmer until the butter has melted then beat in the flour, sugar and salt.
Cook for a couple of minutes to cook out the flour, then off the heat, beat in the eggs, one at a time, until smooth and glossy.
Cool briefly then spoon into a piping bag fitted with a medium size nozzle.
Pipe in 12.5cm/5in long lines onto a baking sheet.
Place in the oven and bake for 20-25 minutes until golden and crisp.
Remove and cool on a wire rack.
Melt the dark chocolate with two tablespoons water in a saucepan over a gentle heat.
Sieve the icing sugar and cocoa into a bowl, then add the melted chocolate and another two tablespoons of water and beat until mixed and thickened.
Spoon the chocolate mixture into a piping bag. Set aside at room temperature until needed.
Whip the double cream until firm peaks form when the whisk is removed, then place into a piping bag fitted with a small nozzle.
Make a small hole in the base of the éclair, and pipe in the cream.
Pipe the fondant chocolate on top of the éclair and set aside to set.
Serve straight away or set in the fridge until needed.
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Featuring recipes by Richard Bertinet and Jonray and Peter Sanchez-Iglesias.