Preheat the oven to 425°F. Lightly grease (or line with parchment) two baking sheets.
To make the éclair shells: Combine the water, butter, and salt in a medium-sized saucepan, heat until the butter has melted, and bring to a rolling boil.
Remove the pan from the heat, and add the flour all at once, stirring vigorously.
Return the pan to the burner and cook over medium heat, stirring all the while, until the mixture smooths out and follows the spoon around the pan; this should take less than a minute.
Remove the pan from the heat, and let the mixture cool for 5 to 10 minutes. It'll still feel hot, but you should be able to hold a finger in it for a few seconds. If you have a digital thermometer, the temperature should be below 125°F.
Transfer the mixture to a mixer, and beat in the eggs one at a time; it'll look curdled at first, but when you add the last egg it should become smooth. Beat for at least 2 minutes after adding the last egg.
If you have a pastry bag, pipe the batter into 5" logs about 3/4" in diameter. Or simply use a spoon or cookie scoop to drop small mounds of the batter in a 5" line, gently spreading them into one solid pastry.
Bake the pastries for 15 minutes, then reduce the oven temperature to 350°F and bake for an additional 25 minutes, until pastries are a medium golden brown. Don't open the oven door while the pastries are baking.
Remove the pastries from the oven. Make a small slit in the top of each, and return them to the oven for 5 minutes, to allow the steam to escape. Place them on a rack to cool. When they're cool enough to handle, split each in half to make top and bottom pieces; splitting and exposing the centers to air will help keep them from becoming soggy.
To make the pastry cream: Combine the sugar and cornstarch in a mixing bowl. Whisk in the yolks.
Bring the milk and cocoa just to a simmer in a saucepan set over medium heat.
Pour a quarter of the hot milk/chocolate into the yolk mixture, whisking until incorporated. Return the mixture to the saucepan over medium heat.
Cook, stirring constantly, until the mixture becomes very thick, and just barely starts to boil. Remove the pan from the heat.
Add the chocolate and butter, stirring until melted and smooth.
Transfer the pastry cream to a bowl, and cover with plastic wrap. Cool to room temperature, then refrigerate for about 2 hours.
To make the glaze: Combine the chocolate and corn syrup in a medium bowl.
Heat the cream to simmering, then pour over the chocolate, stirring until melted and smooth. If the glaze seems thin, let it cool and set a bit before using.
To assemble the éclairs: Pipe or spoon the cream into the pastries, then dip the tops of each one into the glaze. For best results, serve immediately; or refrigerate and serve within several hours.
Store any leftover éclairs, well-wrapped, in the refrigerator for a few days. They don't freeze well, so enjoy éclairs while they're fresh!