In a deep pan, boil 1 cup of water with a pinch of salt and just enough citric acid to fit on the tip of a knife. Add the butter and allow it to melt. While whipping, add the flour and stir well until it begins to separate from the pan.
The pan is pulled off the fire and one by one by the 4 eggs are mixed in. Using a special form or by shaping them with a wet tablespoon, form éclairs, then arrange them in an oiled tray. Put the tray in a highly preheated oven.
Once the éclairs swell and become golden, lower the heat gradually until it's finally off completely, but don't remove the tray until the éclairs cool or they'll deflate!
For the filling, make a milk-egg or classic Bavarian cream. I make the milk-egg cream as follows: I bring some of the milk and the sugar to a boil. With the rest of the milk, dilute the flour and eggs into a mush. Pour it in a thin stream, while mixing, into the pot of sweetened milk that's been removed from the stove.
The mixture is then returned on the heat, stir on low heat until it thickens and makes bubbles. Let the cream cool, while stirring occasionally so a crust doesn't form on top, and in the meantime flavor it with the vanilla.
Cut the cooled eclairs carefully and fill them with the cream. If you have a pastry bag, you can fill them with cream without cutting them. You can sprinkle the éclairs with powdered sugar or decorate them with melted chocolate.