It appears that éclairs were first made by Antonin Carême in the 19th century, the famous French chef. The word comes from French éclair 'flash of lightning', so named because it is eaten quickly (in a flash). True.
Bring water, milk, butter, sugar and salt to a boil. Remove from heat and quickly stir in flour. Cook over medium-high heat, stirring constantly, until mixture pulls away from sides for about 2 minutes. Transfer to the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed for about 1 minute to get the steam out and throw in eggs and mix until smooth. Pipe eclairs or religieuses and heads on a baking sheet lined with a silicon mat or parchment using a 5/8’’/0.625’’/ 15.8 mm open star pastry tip. The use of a star piping tip (or nozzle) to pipe pâte à choux is essential to allowing the batter to expand evenly with minimal cracking during the baking process. The ridges created by piping the pâte à choux with a star tip creates gaps that allows the choux to expand evenly during baking. Dust eclairs and religieuses with powdered sugar before baking. Raw pate a choux can be refrigerated a couple of days or frozen for weeks before being baked. Place frozen eclairs or religieuses on a baking tray and bake right away in the preheated oven.
Set the oven rack adjusted to the middle position. Bake one sheet at a time ~ 10 eclairs or choux.
If using a conventional oven: Bake in a preheated 450ºF/230ºC oven for 5 minutes, then lower temp to 350ºF/180ºC and continue baking for 30 minutes more. Do not open the oven door during baking or it will deflate. Then, turn oven off leaving the oven door ajar to allowing pate a choux to dry for 15 minutes. Remove from oven and let cool completely on wire rack. Lower baking time if using a convection oven.
For the chocolate mixture: bring milk to a boil and pour in the chopped chocolate and cocoa powder. Blend and set aside. For the custard: Bring milk, sugar and vanilla to a boil. Meanwhile combine egg yolks with sugar and cornstarch. When the milk is simmering, remove from the heat and temper the yolk mixture with the hot milk. Pour the tempered mixture into the saucepan and return to the heat; bring custard to a boil and cook for 2 minutes whisking swiftly on medium heat. Remove from the stove and mix in the chocolate mixture and pass through a fine mesh-sieve or mix with an immersion blender. Cool custard over a frozen tray lined with plastic wrap, and top with wrap in contact and chill completely. When ready to use, beat the chocolate custard until smooth. Make holes on the backs of pâte à choux; skip this step if using a Bismarck tip. Fill a pastry bag fitted with a small plain pastry tip or a Bismarck tip, gently pipe the custard generously into the eclairs or choux and with the tip of a knife, wipe any cream excess that might have escaped from the pâte à choux. Chill prior glazing.
In a pastry bowl and over water bath, melt chocolate slowly. Meanwhile, heat up heavy cream and whisk in cocoa powder. Pour over melted chocolate and blend to smooth without incorporating too much air. Use chocolate glaze at 105ºF/40ºC.
Dip the tops of the chilled eclairs or choux in the warm chocolate glaze and set let on a sheet pan. To finish the religieuses, chill remaining chocolate glaze and whip until light and fluffy. Using a very small open start pastry tip, pipe small drops around the neck of the religieuses. Buttercream is commonly used for that effect. Eclairs or religieuses can be refrigerated up to 3 days. Do not freeze. Enjoy!