For the Filling:

1. In a medium saucepan, combine the milk with the vanilla bean and heat over medium until boiling. Once boiling, turn off the heat and allow it to infuse for 15 minutes. Remove the vanilla bean pod afterward.
2. In a mixing bowl, whisk together the egg yolks and sugar until they become light and fluffy, which should take about 1 minute. Incorporate the cornstarch and whisk thoroughly until smooth. Gradually whisk in ¼ cup of the hot milk mixture until well blended, then add the rest of the hot milk mixture while reserving the saucepan. Strain the mixture through a fine mesh strainer back into the saucepan.
3. Cook over medium-high heat, whisking gently but constantly, until the mixture thickens and begins to boil slowly, taking about 3 to 5 minutes. Remove from heat and mix in the butter until fully melted and combined. Transfer to a clean bowl and cover the surface of the pastry cream with plastic wrap to avoid a skin forming. Allow it to cool slightly at room temperature, then refrigerate for at least 2 hours or until ready to use. The custard can be prepared up to 24 hours in advance.

For the Pastry:

1. Preheat the oven to 425°F. On one side of a sheet of parchment paper, use a pencil to draw ten 4-inch-long lines, spacing them a few inches apart. Flip the parchment over and place it on a large baking sheet. Prepare a large pastry bag fitted with a large round tip and set aside.
2. In a large saucepan, combine the water, butter, sugar, and salt, then bring to a rolling boil over medium-high heat, which should take about 5 minutes. Once boiling, remove the pan from the heat. Using a wooden spoon, add all the flour at once and stir vigorously until fully incorporated, for about 30 to 60 seconds.
3. Return the pan to medium heat and stir constantly for an additional 30 seconds. Transfer the mixture to the bowl of a mixer fitted with a paddle attachment (or use a hand mixer).
4. Begin mixing at medium speed. While mixing, add 3 eggs one at a time, stopping to scrape down the sides of the bowl after each addition. Continue mixing until the dough is smooth and glossy, with the eggs completely incorporated. The dough should be thick and fall slowly from the paddle when lifted. If it clings to the paddle, beat the remaining egg and gradually mix it in until the dough is thick yet can ribbon off the paddle.
5. Carefully transfer the choux dough into the pastry bag. Pipe thick lines of dough approximately 4 inches long onto the parchment-lined baking sheet, ensuring to leave 2 inches of space between each line. You should have 8 to 10 lines of dough. (Perfection is not necessary!)
6. Using fingers dipped in cool water, smooth out any bumps or points on the dough surface. Optionally, brush the dough lightly with an egg wash.
7. Bake for 15 minutes, then reduce the oven temperature to 375°F and continue baking until the pastries are puffed and a light golden brown, which should take about 25 minutes more. Avoid opening the oven door frequently while baking. Once golden, remove from the oven and immediately poke two holes in the bottom with a skewer to release steam. Allow the pastry shells to cool completely on a wire rack.

For Filling the Shells:

1. Make a hole at the end of each cooled eclair shell, or create two larger holes on the bottom using a small pastry tip.
2. Take the pastry cream from the refrigerator and whisk it vigorously until smooth and loosened. Transfer it to a pastry bag fitted with a medium plain piping tip or a tip suitable for filling donuts. Carefully pipe the custard into the eclairs through the holes, filling them just enough to prevent overflow. Once filled, place them back on the wire rack.

For the Glaze:

1. In a small saucepan over medium heat or in a microwave-safe bowl, heat the cream until it begins to bubble and steam.
2. Add the chocolate and let it sit for 2 minutes. Whisk gently until the mixture is melted and smooth.
3. Dip the tops of the eclairs into the warm chocolate glaze and place them on a small pan or plate. Chill uncovered for at least 1 hour to allow the glaze to set, or keep refrigerated until serving. Store eclairs in an airtight container in the fridge for up to 2 days.