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Preheat the oven to 170C/325F/Gas 3.
For the choux pastry, place the water, milk, butter, sugar and salt in a medium saucepan over a high heat and bring the mixture to a boil.
Remove the pan from the heat and, using a wooden spoon, quickly beat in the flour until the mixture is completely smooth.
Turn the heat down to medium, return the pan to the hob and cook for about one minute, beating all the time, or until the mixture comes away from the edge of the pan.
Remove the pan from the heat and gradually beat in the eggs until you have a smooth paste.
Transfer the paste to a large piping bag fitted with a 1.5cm/½in fluted nozzle and let the mixture to cool for about five minutes in the bag to stiffen slightly before you begin the piping.
Line a large baking tray with greaseproof paper and pipe on 12 éclairs, each about 15cm/6in long. Alternatively, for a more professional and uniform finish, pipe four rows of pastry, each about 36cm/14in long, onto a non-stick tray and freeze. Cut the frozen strips into three and either defrost and cook as below, or bake from frozen and add five minutes to the cooking time.
Bake the éclairs in the preheated oven for 30-35 minutes or until golden-brown, then transfer to a rack and leave to cool.
For the filling, melt the chocolate in a bowl over a pan of simmering water. Do not let the bottom of the bowl touch the water or the chocolate will burn.
Pour the melted chocolate into the pastry cream, mix in the cocoa powder and whisk together to a smooth consistency. You could chill the mixture at this point for 30 minutes, then whisk again before you fill the éclairs, to give a creamier mouth-feel to the filling.
When you are ready to fill the éclairs, transfer the filling to a piping bag fitted with a 0.5cm/¼in nozzle. Pierce the underside of each éclair four times with the tip of the nozzle, gently squirting a little of the filling into the éclair as you do so.