Preheat your oven to 425°F (220°C). Line two baking sheets with parchment paper. Fit a piping bag with a large star or round tip.

In a heavy-bottomed saucepan, combine the water, diced butter, granulated sugar, and salt. Bring to a rolling boil over medium-high heat, stirring occasionally to ensure the butter melts completely.

Once boiling, remove the pan from the heat and immediately add the sifted all-purpose flour all at once. Stir vigorously with a wooden spoon or rubber spatula until the mixture forms a ball and pulls away from the sides of the pan. Continue to cook over medium heat for 1-2 minutes, stirring constantly, to dry out the dough; a film should form on the bottom of the pan.

Transfer the dough to the bowl of a stand mixer fitted with a paddle attachment (or use a large bowl and a hand mixer). Beat on low speed for 1-2 minutes to cool the dough slightly. This prevents the eggs from scrambling.

With the mixer on medium-low speed, gradually add the lightly beaten eggs, a little at a time, allowing each addition to be fully incorporated before adding more. The dough will look curdled at first but will come together to a smooth, glossy, thick batter. The finished dough should be soft enough to pipe but hold its shape.

Transfer the choux pastry to the prepared piping bag. Pipe 4-5 inch long éclairs onto the parchment-lined baking sheets, leaving about 2 inches between each. You should get 12-14 éclairs.

Place the baking sheets in the preheated oven. Bake at 425°F (220°C) for 10 minutes. Without opening the oven door, reduce the temperature to 375°F (190°C) and continue baking for another 25-30 minutes, or until the éclairs are puffed, golden brown, and feel light and hollow. Do not open the oven door too early, or the éclairs may collapse.

Once baked, turn off the oven and leave the éclairs inside with the oven door slightly ajar for 5-10 minutes to dry them out further. This helps prevent them from becoming soggy. Remove from the oven and transfer to a wire rack to cool completely.

While the éclairs cool, in a medium saucepan, combine the whole milk and the scraped vanilla bean seeds (and pod, if using). Heat over medium heat until the milk just begins to simmer around the edges.

In a separate medium bowl, whisk together the egg yolks, half of the granulated sugar (1/4 cup), and cornstarch until smooth and pale.

Remove the vanilla bean pod from the hot milk. Gradually temper the hot milk into the egg yolk mixture by slowly pouring a small amount of milk into the yolks while whisking constantly. Once about half of the milk has been added, pour the entire mixture back into the saucepan.

Cook over medium heat, whisking constantly, until the cream thickens significantly and comes to a boil. Boil for 1 minute while whisking vigorously to ensure the cornstarch is fully cooked. Remove from heat.

Stir in the diced cold unsalted butter until fully melted and incorporated. If desired, strain the pastry cream through a fine-mesh sieve into a clean bowl for an extra smooth texture.

Press plastic wrap directly onto the surface of the pastry cream to prevent a skin from forming. Refrigerate for at least 1 hour, or until thoroughly chilled and firm.

In a heatproof bowl set over a small saucepan of simmering water (a double boiler), or in a microwave-safe bowl, combine the finely chopped dark chocolate, heavy cream, unsalted butter, light corn syrup (if using), and a pinch of salt. Stir occasionally until completely melted and smooth.

Keep the glaze warm over very low heat or in the residual heat of the double boiler while you fill the éclairs.

Once the éclairs are completely cool and the pastry cream is chilled, transfer the pastry cream to a piping bag fitted with a small round tip (or a sharp knife to make a small hole at the bottom of the éclair).

Carefully poke 2-3 small holes on the underside of each éclair shell. Insert the piping tip into each hole and pipe the pastry cream until the éclair feels heavy and plump.

Dip the top of each filled éclair into the warm chocolate glaze, allowing any excess to drip off. Place them on a wire rack or parchment paper.

Allow the glaze to set for about 15-20 minutes at room temperature, or briefly in the refrigerator, before serving. Enjoy your homemade Chocolate Éclair Dessert!