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1
Line two baking sheets with parchment paper. Using a pencil and ruler, lightly mark eighteen 1-by-5-inch rectangles as templates for piping the eclairs. Fit a pastry bag with a medium (1/2- or 5/8-inch) round tip.
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2
Preheat the oven to 425u00b0F. In a medium saucepan, combine the milk, water, butter, sugar, and salt. Bring the mixture to a boil over medium heat.
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3
Using a wooden spoon, stir in the flour and stir energetically over low heat until the dough is very thick and pulls away from the sides of the pan, about 3 minutes. Some of the moisture should evaporate.
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4
Transfer the dough to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed for 1 minute, then begin adding the eggs, one at a time. Stop and scrape down the sides of the bowl as needed. Once the eggs are all added, raise the speed to high speed and beat for 1 minute to add a little extra air to build structure.
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5
Transfer some of the dough to the pastry bag. Carefully pipe tall strips of dough to fit the templates. The strips should be about 3/4 inch tall so that they rise sufficiently. When they are all piped, brush each eclair with egg wash using a pastry brush. Gently brush the back of a fork along each eclair to make grooves down the length of them. This helps them bake evenly. You can scrape up and re-pipe any mistakes, but they will not rise quite as well.
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6
Bake the dough for 15 minutes without disturbing. Turn off the oven and leave the eclairs inside until they are browned and feel light and hollow, 20 to 25 minutes longer. Immediately pierce each end of the eclairs with the large end of a chopstick to release the steam. Transfer the eclairs to wire racks and let cool completely.
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7
To finish the eclairs, fit a pastry bag with a 1/4-inch round tip and fill each with pastry cream that has been whisked smooth. Carefully fill the eclairs with pastry cream, piping it into the hole on the end without letting it seep out, but making sure there is pastry cream throughout the whole eclair.
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8
Warm the ganache until it flows easily. You can add some warm water to it to thin it out. Pour the ganache into a shallow bowl. Dip the top of each filled eclair in the warm ganache, letting the excess drip off. Set the finished eclairs on wire racks to dry. Once the ganache is dry, serve the eclairs immediately or refrigerate for up to 3 hours.
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9
Pour the milk into a medium pot. Use a sharp knife to slit the vanilla bean lengthwise and scrape the seeds into the milk. Put the bean in the milk as well. Heat the milk until almost boiling (bubbles will begin to form at the edges). Cover and let steep for 1 hour if time permits, otherwise proceed as directed.
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10
In a medium bowl, whisk together the yolks, sugar, and cornstarch until smooth. Set the bowl on a kitchen towel or nonskid surface and whisk the eggs while pouring about 1/2 cup of the hot milk into them to temper. Gradually pour in the rest of the milk, whisking constantly. Pour the contents of the bowl into the pan and set over medium-low heat.
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11
Cook, whisking constantly, until the mixture thickens and comes to a slow boil, about 2 minutes. Immediately strain the pastry cream through a fine-mesh sieve into a clean container. Discard the vanilla bean or wash and reuse it. Let the pastry cream cool to room temperature, 10 minutes, and then whisk in the butter. You want the butter to be incorporated without being melted.
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12
Cover the pastry cream with plastic wrap, pressing it directly on the surface of the cream to prevent a skin from forming. Refrigerate until well chilled, at least 1 hour and up to 3 days.
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13
Combine the chocolate and powdered sugar in a heatproof bowl. In a saucepan over medium heat, bring the cream to a gentle simmer. Pour the hot cream over the chocolate and stir until the sugar is dissolved. Nest the bowl over a pan of simmering water to make a bain-marie. Heat, stirring, until all of the chocolate is melted and the mixture is smooth. Remove the bowl from the heat.
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14
Whisk the egg yolks together in a small heatproof bowl. Pour about 1/2 cup of the melted chocolate mixture into the yolks while whisking, to temper them. Pour the tempered egg-yolk mixture back into the pan of chocolate and whisk to combine. Add the butter and stir until smooth. Pour the hot ganache through a fine-mesh sieve into a clean heatproof bowl.
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15
Use the ganache immediately, or transfer to an airtight container and refrigerate for up to 2 weeks. Ganache does not freeze well.
