Bring the water, butter, salt and sugar to the boil, reduce the heat. Mix the flour and cocoa powder, add all at once, stir with a risotto spoon for approx. 2 mins. until a smooth dough is formed which separates from the base of the pan. Remove the pan from the heat. Using the dough hook on a hand mixer, gradually fold in the eggs; the dough should be soft but not runny.
To shape the éclairs
Spoon the mixture into a piping bag with a serrated nozzle (approx. 14 mm in diameter). Pipe 30 éclairs (approx. 8 cm long) onto a baking tray lined with baking paper, ensuring they are well spaced out.
Approx. 25 mins. in the centre of an oven preheated to 180°C. Do not open the oven during baking! Then switch the oven off and leave to dry for approx. 15 mins. with the door slightly ajar. Remove, leave to cool on a rack, cut the éclairs in half crosswise using scissors.
Light chocolate mousse
Heat the cream in a pan, remove the pan from the heat. Add the chocolate, melt, stir until smooth, then leave to cool. Whip the cream until stiff, fold in, cover and chill for approx. 2 hrs. Transfer the mousse to a piping bag with a smooth nozzle (approx. 1 cm in diameter).
Dark chocolate ganache
Heat the cream in a pan, remove the pan from the heat. Add the chocolate, melt, and stir until smooth. Cover the ganache and leave to cool, transfer to a piping bag with a smooth nozzle (approx. 5 mm in diameter). Pipe the ganache and mousse in alternate layers over the base of the éclairs.
Melt the chocolate in a thin-sided bowl over a gently simmering bain-marie, stir until smooth. Glaze the top half of each éclair with chocolate, place on top of the bottom half.