1. Craft the Creamy Pastry Filling:
In a medium bowl, simply whisk together the granulated sugar, egg yolks, cornstarch, and kosher salt until it's super smooth. No lumps allowed!
Now, in a saucepan, gently warm the whole milk, heavy cream, and the fragrant vanilla bean seeds (or extract) over medium heat until it just starts to bubble.
Gradually pour about half of that warm milk mixture into your egg yolk bowl, whisking like crazy to slowly bring the egg mixture up to temperature without scrambling it.
Pour this tempered egg mix back into the saucepan with the rest of the milk. Cook over medium heat, stirring constantly, until the cream gets wonderfully thick and boils gently for about a minute.
Take it off the heat and stir in the little cubes of unsalted butter until they've completely melted and vanished into the cream.
For an extra-smooth finish, push the pastry cream through a fine-mesh sieve into a clean bowl. Cover it with plastic wrap pressed directly onto the surface to prevent a skin, then pop it in the fridge for at least 4 hours, or ideally overnight, until it's perfectly chilled and firm.
2. Bake the Puffy Choux Pastry:
Preheat your oven to a cozy 400°F (200°C) and line a large baking sheet with parchment paper.
In a medium saucepan, combine the whole milk, unsalted butter, and fine sea salt. Bring this mixture to a lively boil over medium-high heat.
Remove the pan from the heat and quickly add all the flour in one go. Stir vigorously with a wooden spoon until the dough forms a neat ball and pulls away from the sides of the pan.
Return the pan to medium heat and continue to cook the dough, stirring for another 2-3 minutes to dry it out. You'll know it's ready when you see a thin film at the bottom of the pan.
Transfer the dough to a stand mixer bowl (or a large bowl if you're mixing by hand). Let it cool down for a few minutes.
Add the eggs one at a time, making sure each egg is fully mixed in before adding the next. The dough will transform into a beautiful, smooth, and glossy consistency. It should form a soft peak when you lift your spoon. If it feels too stiff, just add a bit of a whisked extra egg, a little at a time, until it's just right.
Spread the choux dough evenly onto your prepared baking sheet. Don't stress about making it perfect – it's all going to be covered up later!
Bake for 30 minutes, then lower the oven temperature to 350°F (175°C) and bake for another 20-25 minutes, or until the choux is golden brown, impressively puffed, and sounds hollow when gently tapped. Let it cool completely on a wire rack.
3. Whip Up the Rich Chocolate Ganache:
In a small saucepan, warm the heavy cream and kosher salt over medium heat until it just begins to simmer.
Take it off the heat and pour the hot cream over your chopped dark or semisweet chocolate in a heatproof bowl. Let it sit undisturbed for 5 minutes to melt the chocolate.
Gently whisk until the ganache is wonderfully smooth and shiny. Let it cool slightly until it's pourable but not piping hot.
4. Assemble Your Amazing Éclair Cake:
Once your choux pastry is completely cool, carefully slice it horizontally into two equal layers using a serrated knife.
Place the bottom layer of the choux pastry on your favorite serving platter.
Grab that chilled pastry cream from the fridge and give it a quick whisk to loosen it up. Spread all of the creamy goodness evenly over the bottom choux layer.
Gently set the top choux layer over the pastry cream.
Finally, pour the slightly cooled chocolate ganache over the top layer, spreading it out evenly to cover every inch.
Refrigerate your magnificent éclair cake for at least 2 hours. This helps it set beautifully and allows all those delicious flavors to mingle.
Slice and serve your easy, impressive Éclair Cake!