Start with filling. Take a medium saucepan and heat your milk and vanilla bean to boil over medium heat. Turn of the heat immediately and set aside for 15 minutes. Take a separate bowl and whisk the egg yolks and sugar until light. Take the cornstarch and add to the mixture before whisking until no lumps remain.

Whisk 1/4 cup of hot milk mixture until incorporated. Whisk the remaining milk mixture and reserve the saucepan, then pour through a strainer back to the saucepan. Cook over medium-high heat and whisk until thick. Remove from heat and stir in butter. Let cool. Cover with plastic wrap, press against the plastic to keep from skin forming. Chill for 2 hours.

Now the pastry. Preheat oven to 425 degrees F. Line sheet pan with parchment paper. Bring water butter salt and sugar to a rolling boil in a saucepan over medium-high heat. Once boiling, take off heat. Stir with a wooden spoon and add the flour. Stir until flour is incorporated 30 - 60 seconds. Put back on heat and cook and stir for 30 seconds. Scrape mixture into a mixer with paddle attachment or use a hand mixer. Mix at medium speed, add 3 eggs, 1 at a time. Mix until dough is smooth and eggs are incorporated. Dough should be think and should fall slowly. If it is still clinging to beaters, add additional egg.

Use a pastry bag with large plain tip, and pipe fat lengths of dough onto baking sheet. Leave 2 in between them.

Now the egg wash. Take a bowl and whisk egg and water together. Brush surface or each eclair with the wash. Smooth any bumps with fingers. Bake for 15 minutes, reduce heat to 375 degrees then bake until puffed up. About 25 more minutes. Let cool on baking sheet. Fit a medium sized plain pastry tip over your finger and make a hole in each eclair. Using an icing back, pipe custard filling into eclairs. Don't stuff em full, just enough.

For the glaze, take small saucepan, and heat cream over medium heat until it boils. Put chocolate in medium bowl. Pour hot cream over chocolate and whisk until smooth. Set aside and keep warm. If made 48 hours in advance, cover and refrigerate until ready to use. Rewarm in microwave or over hot water when ready.

Dip tops of eclairs in chocolate glaze and set on sheet pan. Chill uncovered for 1 hour at least. Serve chilled.