Put the water, milk, salt, butter and sugar into a saucepan and bring to a boil. Remove from the heat, add all the flour at once and stir with a wooden spoon until smooth. Put back in the hob and stir until it comes if the sides of the saucepan.
Let cool down a bit, and beat in the eggs one by one until you have a smooth paste. Cool down, and pipe into either small balls (5cm) or eclairs.
Bake for 10 minutes at 220 Celsius and a further 10 minutes at 180 Celsius.
In a large mixing bowl, whisk together the eggs and sugar until they turn a pale blonde colour.
Whisk in the flour and cornflour and set aside.
Place the milk and vanilla syrup in a heavy-bottomed saucepan, bring to a boil and simmer for about five minutes.
Remove the pan from the heat and let cool for 30 seconds.
Pour the hot milk onto the egg mixture, whisking all the time, then return the mixture to the pan. It is important to pour the hot milk onto the cold eggs before you return the mixture to the pan to prevent the eggs from scrambling.
Bring the mixture back to the boil and simmer for one minute, whisking continuously, or until smooth.
Pour the cream into a clean bowl and dust with icing sugar to prevent a skin forming.
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.
For the glaze, gently warm the fondant in a small pan over a low heat until it reaches body temperature.
Stir in the cocoa powder and enough of the water to make a smooth paste, then transfer to a piping bag fitted with a 1.5cm/½in nozzle.
Pipe the glaze onto the top of each éclair, removing any excess with the back of your finger, then place in the fridge for the glaze to set before serving.