MAKE THE PASTRY:
In a small sauce pan, bring water and butter to a boil over medium heat. Add flour, milk and salt all at once. Beat with a hand mixer until blended and dough becomes solid. It will look gummy and a few butter will separate. This is normal.
Remove from heat and let stand for 10 minutes to cool.
Add eggs, one at a time, beating with a hand mixer after each addition. It will look chunky but keep beating. It will smooth out and will look shiny eventually. That’s when you know it’s done.
Insert a piping bag fitted with a large plain tip inside a tall glass/container. Transfer the smooth dough into the bag.
Pipe 3 - 4 inch fat lengths into a parchment lined baking sheet spacing about 2’ apart. Cut each piping with scissors. Smooth the top with the egg wash Use your fingers to smooth out any bumps.
Bake until puffed, very firm and golden brown. You can check by opening the eclairs. It should be hard and hollow inside. Poke 3 holes a the bottom of the baked eclairs let the steam out. Cool on wire rack.
If using vanilla bean: In a medium saucepan, heat the milk and vanilla bean to a boil over medium heat. Immediately turn off the heat and set aside to infuse for 15 minutes.
In a bowl, whisk the egg yolks and sugar until light lemon in color and fluffy. Add cornstarch and whisk vigorously until no lumps remain. Whisk in 1/4 cup of the hot milk mixture until incorporated. Whisk in the remaining hot milk mixture, reserving the saucepan.
Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and starting to boil. Remove from the heat and stir in the butter. Let cool slightly.
Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Chill at least 2 hours. The custard can be made up to 24 hours in advance. Before using, whisk to smooth out the custard before use and set aside. In a separate bowl, beat 1/2 cup heavy cream and fold into the chilled custard. Place a piping bag fitted with a small round tip inside a tall glass and pour the filling inside the piping bag.
FOR THE GLAZE:
In a small saucepan, heat the cream over medium heat just until it boils. Immediately turn off the heat.
Put the chocolate in a medium bowl. Pour the hot cream over the chocolate and whisk until melted and smooth.
Set aside and keep warm. The glaze can be made up to 48 hours in advance. Cover and refrigerate until ready to use, and rewarm in a microwave or over hot water when ready to use.
Fill the eclairs with the filling using a piping bag fitted with a small round tip. You can use the same holes you made to let the vent out. Do not over fill.
Dip the tops of the eclairs in the warm chocolate glaze and set on a sheet pan. Chill, uncovered, at least 1 hour to set the glaze. Serve chilled.