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!