Chocolate Éclairs

Preheat oven to 425˚. Line a rimmed baking sheet with parchment paper. Fit a large pastry bag with a plain ½" tip.

Heat milk, butter, sugar, salt, and ½ cup water in a medium saucepan over medium, stirring occasionally to melt butter. As soon as butter is melted, add flour all at once and stir with a wooden spoon, carefully at first to incorporate, then vigorously, until dough forms a single mass. Continue stirring, slapping dough against sides of saucepan with spoon, until dough leaves a thick film on bottom of pan and pulls away from sides, about 3 minutes (the important thing here is to cook the flour and dry out the dough). Transfer to a medium bowl and let rest 3 minutes.

Using a wooden spoon, mix 5 eggs into dough one at a time, making sure to completely incorporate each egg before adding the next. Dough will look broken at first, then homogenized. The final texture should be smooth, glossy, and somewhat stretchy.

Spoon dough into prepared pastry bag. Pipe 4"-long logs of dough onto prepared baking sheet, spacing at least 2" apart.

Beat remaining egg in a small bowl until no streaks remain and lightly brush tops of dough. Bake 15 minutes, then lower heat to 350˚. Continue to bake until shells are deep golden brown all over and sound hollow when tapped on the bottom, 15−20 minutes.

Remove shells from oven; turn off oven. Poke a small hole in both ends of each shell with a paring knife and return to baking sheet. Return to still-hot oven, propping door open with a wooden spoon. Let dry out and cool, 30–35 minutes.

Do Ahead: Dough can be made 1 day ahead. Transfer to pastry bag and chill.

Combine chocolate and espresso, if using, in a medium heatproof bowl set over a saucepan of barely simmering water (do not let water touch bowl). Stir constantly until chocolate is melted; set aside.

Place milk, butter, and salt in a medium saucepan, then scrape in vanilla seeds and pod. Bring to a simmer over medium heat.

Meanwhile, whisk egg yolks and sugar in a medium bowl until very pale and fluffy, about 3 minutes. Add cornstarch, whisking until no powdery dry spots remain. Whisking constantly, gradually add hot milk mixture to egg mixture. Transfer back to saucepan and cook over medium heat, whisking constantly, until mixture is thick and holds whisk marks, about 2 minutes.

Remove pastry cream from heat and whisk in reserved cooled melted chocolate. Set a fine mesh sieve over a medium bowl. Press pastry cream through sieve into bowl. Cover with plastic, pressing directly onto surface. Chill until cold, at least 2 hours.

Fit another large pastry bag with a star tip. Transfer chilled pastry cream to bag. Working one at a time, gently pipe pastry cream into holes at each end to fill.

Do Ahead: Pastry cream can be made 3 days ahead. Transfer to pastry bag and chill.

Bring sugar, corn syrup, and ¼ cup water to a boil in a small saucepan over medium heat, stirring to dissolve sugar. Clip thermometer to side of pan and cook, swirling pot (do not stir) and brushing down sides with a wet pastry brush, until thermometer registers 238˚ (also known as soft-ball stage). Immediately pour into a food processor and let sit until thermometer registers 140˚.

Process until syrup is thick and white, 1–2 minutes. Working quickly because fondant will harden, scrape into a medium heatproof bowl set over barely simmering water (do not let water touch bowl). Add melted chocolate, cocoa powder, and 2 Tbsp. water; stir until smooth and any hardened bits of fondant are melted. Continue adding water ½-teaspoonful at a time until mixture is a pourable consistency (you’re looking for a ribbon that falls back onto itself and quickly dissipates, leaving a smooth surface).

Invert filled éclairs and dip tops in fondant, letting excess drip off before quickly transferring back to baking sheet, glazed side up (fondant will begin to set quickly). Let sit until fondant is no longer wet.