Black & White Éclairs

Directions

1. Line two baking sheets with parchment paper. If desired, you can draw lines on the paper 4 inches apart to use as guides for keeping your éclairs the same length (if you do, turn the paper over on the baking sheet so that the ink will not make contact with the batter).
2. Transfer the prepared pâte à choux to a pastry bag (either fitted with large round piping tip or with a ½ inch opening cut from the tip). Pipe a very small amount of batter in the corners of the paper so that it adheres to the baking sheet (this makes it easier to pipe evenly without the paper moving).
3. Apply steady pressure to the pastry bag to pipe 4 inch long éclairs between the guide lines you drew. You can fit multiple rows on your baking sheet, and stagger the éclairs as you pipe. When you have reached the preferred length, stop applying pressure, and use a quick flick of the wrist to break the connection between the batter and the piping tip. Repeat with the remaining batter.
4. Use a fingertip dipped in cool water to smooth the edge where you finished piping, if desired. Let the éclairs sit at room temperature for 15 to 30 minutes. Preheat the oven to 375°F.
5. Brush the éclairs with egg wash. Bake until golden and crisp, 35 to 40 minutes. (Optional: Once the éclairs have become lightly golden and the structure is set, about 30 minutes into baking, you can remove the trays from the oven and let cool completely, then return to the oven for 5 to 10 minutes more. This helps the choux brown more evenly.)
6. When you remove the choux from the oven, immediately use a paring knife to poke a small vent into the side of the éclairs to allow steam to escape (this will help to keep them crisp). Cool completely.
7. Make the fillings: In a medium pot, bring the milk, cream, and salt to simmer over medium low heat. In a medium bowl, rub the sugar and vanilla bean together to combine. Whisk in the cornstarch until well combined, then whisk in the egg yolks.
8. When the milk mixture has come to a simmer, gradually pour about ¼ of it into the egg yolk mixture, whisking constantly to combine. Return this mixture to the pot, whisking constantly over medium-low heat until the mixture begins to thicken. When it does, you can switch to a silicone spatula to stir; keep stirring constantly until the mixture comes to a boil.
9. Remove the custard from the heat and stir in the butter. Strain half of the custard into a medium bowl, and cover directly with plastic wrap. Stir the chocolate into the remaining pudding, then strain it into a medium bowl, and cover directly with plastic wrap.
10. Refrigerate both custards until chilled. Once they are cool, transfer them each to a pastry bag fitted with a Bismarck tip or a medium round pastry tip.
11. Starting with the vanilla filling, insert the tip into the left side of the base of the éclair, and apply pressure until it feels slightly heavier. Repeat with the remaining éclairs. Then, do the same with the chocolate filling, inserting the tip into the right side of the base of the eclair and applying pressure until it feels full and heavy.
12. Make the glaze: In a medium bowl, whisk powdered sugar, corn syrup, vanilla, and whole milk to form an easily spreadable but still thick glaze. Add more milk as needed to achieve the proper consistency.
13. Transfer half of the glaze to another medium bowl, and whisk in the cocoa powder. If necessary, add more milk to match the consistency of the vanilla extract. Use a small offset spatula to spread vanilla icing on half of the filled éclair, then another offset spatula to apply chocolate glaze to the other. Let set for 15 minutes. Serve within 8 hours.