Make the pastry cream first. In a saucepan, heat 2 cups whole milk with half the sugar until steaming. In a bowl, whisk 4 egg yolks, remaining sugar, and 1/4 cup cornstarch until smooth. Temper by whisking in hot milk slowly, then return to the saucepan. Cook over medium heat, whisking, until thick bubbles appear. Remove from heat, whisk in 2 tablespoons butter, 1–2 teaspoons vanilla, and a pinch of salt.

Chill the cream. Spread the pastry cream in a shallow dish, press plastic wrap directly on the surface, and chill until cold. For lighter filling, you can later fold in softly whipped cream.

Heat the oven and prep pans. Set oven to 400°F (200°C). Line two baking sheets with parchment. Have a pastry bag fitted with a 1/2-inch round tip (or a zip-top bag with a corner snipped).

Cook the choux panade. In a medium pot, bring 1/2 cup butter, 1 cup water, 1/2 teaspoon salt, and 1 teaspoon sugar to a boil. Add 1 cup flour all at once. Stir vigorously over medium heat 1–2 minutes until the dough forms a ball and a thin film coats the pan. This dries the dough for better rise.

Beat in the eggs. Transfer the hot dough to a bowl. Let it cool 3–5 minutes. Beat in 4 large eggs one at a time, fully incorporating each before adding the next. The dough should be glossy and pipeable, forming a V-shape when it falls from the spatula. If too stiff, beat in a teaspoon or two of extra egg.

Pipe the shells. Pipe 4–5 inch logs, spacing well. Keep the bag at a slight angle and apply steady pressure. Smooth any points with a damp finger for even puffing.

Bake for structure. Bake at 400°F (200°C) for 15 minutes, then reduce to 350°F (175°C) and bake 15–20 minutes more until deep golden and firm. Don’t open the oven early or they may collapse.

Dry them out. Turn off the oven. Pierce each shell at one end with a skewer to release steam. Return to the switched-off oven with the door cracked for 10 minutes. This keeps shells crisp.

Cool completely. Let shells cool on a rack. Meanwhile, transfer chilled pastry cream to a piping bag fitted with a small tip.

Make the chocolate glaze. Heat 1/2 cup heavy cream to a simmer. Pour over 6 ounces chopped chocolate with a pinch of salt. Let sit 2 minutes, then stir until smooth and shiny. If too thick, add a splash more warm cream.

Fill the eclairs. Poke two small holes on the bottom of each shell. Pipe pastry cream into each hole until the shell feels full and heavy, dividing the cream evenly.

Glaze and set. Dip the top of each filled eclair into the warm chocolate glaze, letting excess drip off. Set on a rack to firm up 20–30 minutes.

Serve. Eclairs are best the day they’re made, once the glaze has set and the filling is chilled but the shell is still crisp.