To make the choux pastry, pour the water into a saucepan and add the butter on medium heat. when the butter melts, bring to a boil. Add in all the flour and set aside.
Stir quickly until the flour has blended in with the liquid and the mixture leaves the sides of the pan and makes a soft ball.
Leave the mixture to cool slightly, start by beating in the eggs. The more air you can incorporate, the lighter is the pastry.
Spoon the paste into a piping bag fitted with a 1cm star nozzle and pipe finger lengths. Before baking, sprinkle over a few drops of cold water.
Bake the eclairs for about 20 minutes on 200c.
Leave to cool on wire rack.
Fill each eclair with custard by inserting the tip of a pipping bag into the eclair in 3 places (the ends and the middle) by squeezing in the custard until it almost starts to come out.
To make icing: Boil water in a saucepan, and melt the chocolate by putting it in a bowl with butter and water. Stir until it melts. Add icing sugar, and stir.
Using a knife smooth the melted chocolate over the top of each eclair and leave it to set.