How to cook
In a pot, put the butter, milk, water, sugar and salt. Stir on moderate heat until the butter melts and mixture starts boiling. At that point, pour in all the flour at once and stir until you obtain a dough that easily comes off the walls. Using a wooden spoon makes it easiest. Lower the heat and continue stirring another 1-2 min.
After it forms into a nice ball of dough, pour it into a large glass bowl and spread it out along the walls. Leave it aside for about 10 min. to cool. The dough needs to be warm but not hot because it'll boil the eggs if it is.
Turn the oven on to 392°F (200 °C) with the fan off or 365°F (185 °C) with the fan on.
To the cooled dough begin to add the eggs 1 by 1. Stir with a wooden spoon until each is fully absorbed before adding in the subsequent one. Depending on how large the eggs are, you may need 1 more or 1 less. The goal is to obtain a smooth and soft dough that doesn't run.
Form the eclairs in a tray covered with baking paper. You can spray them out with a pastry bag or shape them using 2 spoons. If you get any sharp peaks from the dough, squash them with moist fingers so they don't burn.
Put the eclairs in the preheated oven and bake about 30 min (depending on their size). After they gain a vivid golden color, they're ready. Turn oven off and leave door slightly opened (can use a wooden spoon to keep it from closing) and leave them for another 10 min.
Note: Do not open the door before the eclairs are ready, under any circumstances, or they'll collapse!
Take eclairs out onto a metal cooking grid, make a small hole in each one to let the steam escape and so they don't moisten.
Start making the cream while the eclairs are baking.
Beat the egg yolks, 1/2 the sugar and the flour in a bowl. Put the milk with the rest of the sugar and vanillas on moderate heat. Once heated up well, pour in 3-4 tbsp of it to the egg yolk mixture.
Wait for the milk to come to a boil again, add in the rest of the egg yolk mixture and start stirring energetically (using a whisk is best) until the cream thickens (about 3 min. total).
Pour the prepared cream into a cold, flat tray - metal, ceramic or glass. Spread it out well and cover with foil, to prevent a crust from forming. Leave it aside to cool well.
Filling the eclairs:
Depending on how large the eclairs are, poke 1-2 holes on their bottom using a sharp knife and fill them using a pastry bag with a thin tip. If you don't have a pastry bag, cut them in 2 and fill each of the halves with cream.
Melt the chocolate in a water bath. You can pour it over the eclairs with a spoon or dip the eclairs in the melted chocolate.
You can eat them right away or put in the fridge for the chocolate to harden and turn crunchy. Store in a container or bag in the fridge.