Cream puff pastry is a versatile batter, ready to bake into pastries or fry into crullers. Eclairs are possibly the most exalted of the pastries. A double baking ensures that the pastry stays crisp and light without a soggy interior.
1½teaspoonsinstant espresso powder dissolved in 1tablespoon boiling water and cooled
Position racks in the center and upper third of the oven and heat to 400°F. Line two baking sheets with parchment paper.
Fit a large pastry bag with a 11/16-inch-wide plain tip such as Ateco Number 809 and fill with the warm dough. Spacing i inch apart, pipe strips of dough, about 4½ inches long and 1 inch wide, onto the sheets. Using a pastry brush, brush the strips with the beaten egg, tamping down any pointed ends of dough at the same time. Do not use so much glaze that it drips onto the parchment, or the puffs won’t rise well.
Bake until the eclairs are puffed, golden brown, and set, about 20 minutes. Remove from the oven and pierce each eclair on its side with the tip of a small sharp knife to release the steam. Return to the oven, switching the positions of the sheets from top to bottom and front to back, and continue baking until the eclairs are crisp, about 10 minutes. Cool completely.
Using a serrated knife, split the eclair strips in half lengthwise, separating the tops from the bottoms, but keeping track of the matching halves. Place the tops on a large rack set over a baking sheet.
Make the coffee icing (it must be warm). Pour the warm icing over the eclair tops, letting the excess run off. If you wish, scrape up the icing on the pan and pour over the eclairs to give another coating. Refrigerate until the icing sets, about 10 minutes.
To make the filling: Sprinkle the cornstarch over ½cup of the milk in a medium bowl and whisk to dissolve. Add the sugar and yolks and whisk well. In a heavy-bottomed medium saucepan, bring the remaining 1cup milk to a simmer over medium heat, stirring to dissolve the sugar. Whisk the yolk mixture into the saucepan. Cook, whisking often, until the pastry cream comes to a full boil. Strain through a wire sieve into a large bowl set in a larger bowl of ice water and let stand, stirring often, until chilled.
Whip the cream in a chilled medium bowl with a handheld electric mixer on high speed until stiff. Beat in the dissolved coffee. Stir one fourth of the whipped cream into the pastry cream, and fold in the remaining cream.
Fill a large pastry bag fitted with a 9/16-inch-wide French star tip (such as Ateco Number 865) with the filling. Pipe the filling into the bottom halves of the pastry. Cover with the glazed tops. Refrigerate uncovered until serving. Serve chilled.