Warm the milk in a medium saucepan on low heat just until it begins to barely boil.
Meanwhile, whisk the egg yolks with the sugar for about 1 minute or until light colored and slightly thickened. Whisk in the flour.
Remove the simmering milk from the heat and add it very slowly to the egg yolk mixture, whisking the yolks all the time. Gradually whisk in all of the hot milk. Return the mixture to the saucepan and place on low to medium heat.
Heat mixture until it just boils and has considerably thickened, stirring with a wooden spoon the whole time. This step takes about three minutes and you have to be careful not to scorch the bottom of the mixture.
Remove cream from the heat and immediately pour into a bowl. Stir in vanilla extract and cover the top of the cream with plastic wrap (this keeps it from forming a skin). As soon as it has cooled sufficiently, place cream in refrigerator and chill thoroughly - over night if you wish.
For the choux pastry:
1 cup water
6 tablespoons butter, cut in 6 pieces
1 teaspoon sugar
1/2 teaspoon salt
1 cup flour
4 medium eggs
Set the oven to 425°F (210°C).
Place the water, butter, sugar and salt in a saucepan on medium heat. Heat just until the water boils and the butter has melted.
Remove from the heat and add the flour all at once. Stir with a wooden spoon for at least two minutes. The dough will form a ball, which will then dry out some from the residual heat of the pan. Add the eggs in one at a time, beating vigorously to fully incorporate each egg before adding the next one. The final dough should be stiff and sticky.
Fill a pastry bag equipped with a 3/4 inch tip with the choux paste. Pipe 4.5 inch (11 cm) long by 1.5 inch (3.5 cm) thick logs onto a baking sheet covered with parchment paper, leaving at least an inch between the logs. The recipe should yield 10 to 12 logs.
Bake at 425°F (210°C) for 20 minutes, then reduce the oven heat to 375°F (190°C) and bake for 10 more minutes. Turn off the oven and open the oven door so it is ajar. Leave the logs in the oven for 10 minutes. Remove from the oven and allow to cool completely before going on with the rest of the recipe. You can fix the choux pastry logs several hours in advance of serving time and just leave them out at room temperature.
For the icing:
2 tablespoons unsalted butter, cut in small pieces
1/4 cup cream
2 tablespoons dark, unsweetened cocoa powder
1 cup powdered sugar, sifted
1 teaspoon vanilla
Place the butter and the cream together in a saucepan (large enough so that you can dip the pastries in it) and warm until the butter has melted. Remove from heat and stir in the cocoa powder and sifted powdered sugar. Stir to combine completely, then stir in vanilla extract. The glaze is ready to use right away. If it has cooled down and hardened you can warm it gently so that it is fluid enough to ice the pastries.
Dip the tops of the cooled choux pastry logs in the glaze, rolling them just a bit from side to side so that they are well glazed. Return them to a tray and allow the glaze to firm up (about 30 minutes) before filling with pastry cream. You can leave the unfilled glazed pastries at room temperature for several hours until you are ready to serve.
Final assembly:
Fill a pastry tube or syringe (my preference) with the chilled pastry cream. Being careful not to smudge the chocolate glaze, pick up each pastry by its sides. Poke a small hole in the bottom with the tip of the pastry tube and fill with pastry cream. Each pastry gets about 2 1/2 tablespoons of cream. I like to poke two holes - each about a third of the way from the end of the pastry. Place the pastries on a serving plate and serve immediately.
You can also just slice the pastries in two and spoon on the pastry cream into the middle, then replace the top. A little less elegant, but nonetheless tasty.
Eclairs are best if they are served fresh, but you can refrigerate the assembled pastries for up to a day.
