Classic French eclairs made with airy choux pastry, filled with silky vanilla pastry cream, and finished with a glossy, rich chocolate ganache for an elegant, bakery-worthy bite.
Make the pastry cream: In a large heatproof bowl, whisk egg yolks and cornstarch until thick and smooth.
Heat milk and sugar in a saucepan over medium heat until the sugar dissolves and the mixture reaches a gentle simmer; remove from heat.
Temper the yolks by slowly whisking in the warm milk mixture in a steady stream. Strain back into the saucepan through a fine-mesh sieve.
Cook over medium heat, whisking constantly, until thick and bubbling. Remove from heat and stir in butter, vanilla, and a pinch of salt.
Transfer to a bowl, press plastic wrap directly onto the surface, and chill for at least 3 hours (or up to 24 hours).
Make choux pastry: Prepare 1 batch of choux pastry. Line a baking sheet with parchment and brush the parchment lightly with water.
Pipe 4-inch (10 cm) logs of choux onto the prepared sheet. Smooth any peaks with a damp fingertip and brush the tops lightly with egg wash.
Bake until the shells are deeply golden and puffed, then cool completely on a wire rack.
Make the ganache: Warm the heavy cream until steaming, then pour over the finely chopped chocolate. Let sit briefly, then stir until smooth and glossy. Cool about 15 minutes to reach a good dipping consistency.
Stir the chilled pastry cream to loosen and transfer it to a piping bag fitted with a small round tip.
Fill the shells by poking a small hole in one end and piping in the pastry cream, or split shells horizontally and spread the cream inside.
Dip the tops of the filled eclairs into the ganache. Let set at room temperature (about 1 hour) or refrigerate to speed up setting.