Special equipment: 2 piping bags, plain or blank 5 to 8 mm (1/4 to 3/8-inch) pastry tip, plain or blank 3 mm (1/8-inch) pastry tip, optional
Preheat the oven to 425 degrees F. Line a baking tray with parchment paper.
To make the choux pastry: Place the dough into a piping bag fitted with the plain 5 to 8mm (1/4 to 3/8-inch) tip and pipe into lengths approximately 4-inches long onto the prepared baking tray, spaced about 1 1/2-inches apart to allow for expansion. Use a small wet knife to stop the dough from coming out of the tip when you have finished piping each eclair.
Gently, brush the eclairs with the beaten egg, and bake in the oven for 10 minutes. Reduce the oven temperature to 400 degrees F, and continue to cook for another 15 to 20 minutes, or until the eclairs are puffed up, golden and crisp.
Remove the eclairs from the oven, and using a skewer or the tip of a small sharp knife, make a hole in the side or the base of each eclair. Return to the oven, and bake for another 5 minutes to allow the steam to escape. Transfer the eclairs to a wire rack to cool.
To make the icing: Sift the icing sugar and cocoa powder into a bowl. Add the boiling water, and stir to mix, adding a little more boiling water, if necessary until the icing is spreadable, but not too thin.
To make the creme chantilly filling: Fold the sifted icing sugar and vanilla extract into the whipped cream. Chill the cream until you are ready to use it.
When the eclairs are cool, spoon the creme chantilly into a clean piping bag, fitted with the small, plain tip or use the same tip used for piping the eclairs and pipe the cream into the eclairs through the hole made by the skewer or knife, until they are well filled.
Using a small palette knife or table knife that has been standing in a pot of hot water (to make spreading the icing easier), spread the icing over the top of each eclair, dipping the knife into the hot water between each eclair. Serve and watch them being devoured!
Sift the flour and salt into a large bowl, and set aside.
Place the water and butter in a medium-sized saucepan with high sides (not a low saute pan) set over a medium-high heat, stirring until the butter melts. Bring the mixture to a rolling boil, then immediately remove the pan from the heat. Add the flour, and salt, and beat very well with a wooden spoon, until the mixture comes together.
Reduce the heat to medium and replace the saucepan, stirring for 1 minute until the mixture starts to 'fur' or slightly stick to the base of the pan. Remove the pan from the heat, and allow to cool for 1 minute.
Pour about a quarter of the beaten egg into the pan and, using the wooden spoon, beat very well. Add a little more egg, and beat well again until the mixture comes back together. Continue to add the egg, beating vigorously all the time, until the mixture has softened, is nice and shiny and has a dropping consistency. You may not need to add all the egg, or you may need a little extra. If the mixture is too stiff (not enough egg), then the choux pastries will be too heavy, but if the mixture is too wet (too much egg), they will not hold their shape when spooned onto greaseproof paper.
Although the pastry is best used right away, it can be placed in a bowl, covered, and chilled for up to 12 hours, until ready to use.
Makes about 1 pound 12 ounces