Preheat the oven to 400 degrees. In a medium saucepan, bring the water, butter, sugar and salt to a boil over moderate heat. Remove the pan from the heat. Add the flour and stir vigorously with a wooden spoon for about 2 minutes, until the dough comes together and a film forms on the bottom of the pan. Transfer the dough to a large bowl and beat at medium speed until slightly cooled, about 1 minute. Add the eggs, 1 at a time, beating well after each addition.
Transfer the dough to a pastry bag fitted with a 1-inch round tip. Pipe twelve 5-inch-long logs onto a baking sheet and bake for 10 minutes. Turn the oven down to 325 degrees and bake the shells for 30 minutes longer, or until golden brown. Transfer to a rack and let cool.
In a medium saucepan, bring the milk, vanilla bean and seeds just to a boil. Meanwhile, in a large bowl, whisk the sugar with the cake flour and salt. Whisk in the egg and egg yolks. Slowly add the hot milk, whisking constantly. Pour the mixture back into the saucepan and bring to a boil over moderate heat, whisking constantly. Continue to boil the pastry cream, whisking constantly, until thick, about 30 seconds longer. Immediately strain the pastry cream through a fine sieve into a medium bowl. Cover with plastic wrap and refrigerate for at least 30 minutes, or until cool.
In a medium bowl, whip the heavy cream until soft peaks form. Whisk the pastry cream, then fold in the whipped cream until blended.
In a medium bowl, melt the chocolate in a microwave oven. Whisk in the butter until smooth.
With a serrated knife, split the eclair shells lengthwise. Spoon a generous amount of the pastry cream into the bottom half of each shell. Dip the top half of each shell into the chocolate glaze, close the eclairs and serve.
The components can be made early in the day, but assemble the éclairs shortly before serving.