Preheat the oven to 200C/400F/Gas 6. Line a baking tray with greaseproof paper.
Heat the water and butter in a saucepan until the butter has melted.
Beat in the flour, sugar and salt until the mixture is smooth, then continue to cook for 2-3 minutes. Remove the pan from the heat and beat in the eggs, one at a time, until smooth and glossy.
Set aside to cool, then spoon the dough into a piping bag fitted with a medium size nozzle.
Pipe 12.5cm/5in lines onto the baking tray and bake in the oven for 20-25 minutes, or until golden-brown and crisp.
Remove the éclairs from the oven and set aside to cool on a wire rack.
Meanwhile, heat the dark chocolate and two tablespoons of water in a saucepan until the chocolate has melted.
Sieve the icing sugar and cocoa into a bowl, then add the melted chocolate and water and beat until the mixture is smooth and thickened.
Spoon the mixture into a piping bag.
Whip the double cream in a bowl until stiff peaks just form when the whisk is removed. Spoon the mixture into a piping bag fitted with a small nozzle. Carefully insert the nozzle into the base of the éclairs and fill with cream.
Pipe the fondant chocolate on top of the éclairs and set aside until set.