1) Preheat the oven to 220C/425F/gas mark 7. Lightly butter two to three large baking trays. For the pastry, fold a sheet of greaseproof paper in half, then open it out to create a crease. Sift the 15g flour on to the paper. Place 200ml water, minus 1 tbsp, the salt and butter in a medium saucepan and bring to the boil. When the butter has melted, take the pan off the heat and shoot in the flour, stirring constantly with a pointed wooden spoon.
2) Stir until thick, beating out any lumps. Place the pan back on the heat and cook, beating vigorously, until the dough leaves the sides of the pan and forms a ball in the centre of the pan. Take off the heat and leave to cool for 3-4 minutes.
3) Beat in the eggs one at a time, beating the mixture vigorously until it is smooth and very glossy. Spoon the pastry into a large piping bag fitted with a 1cm plain nozzle. Pipe 12-14 x 7.5-10cm/3-4in lengths on the baking sheets, leaving plenty of room for the éclairs to rise. Use a knife, dipped in cold water, to cut off each length as you pipe.
4) Place the baking sheets in the oven, turn down the heat to 200C/400F/gas mark 6 and cook the éclairs for 20-25 minutes, or until well risen and very brown. They should be light and very crisp to touch. Turn on to a wire tray to cool, and pierce a hole with the tip of a sharp knife in the side of each éclair, to allow moist air to escape.
5) Make the filling. If using the vanilla pod, place with the milk in a saucepan. Bring to the boil, take off the heat and infuse for 30 minutes. Beat the egg yolks with the sugar until pale, then beat in the flours. Bring the milk back to the boil, and stir into the egg mixture. Return to the pan and gently cook, whisking all the time, until thick and smooth. Cook for 4-5 minutes, stirring constantly, then take off the heat and stir in the vanilla extract, if using. Remove the vanilla pod, if used. Turn into a bowl and beat from time to time as it cools. Press a sheet of wetted greaseproof paper over the surface and leave until cold. Beat until smooth, then fold in the cream.
6) Cut the éclairs in half horizontally, and scrape out excess pastry with the tip of a spoon. Spoon in the filling and replace the top.
7) For the icing, melt the chocolate in a bowl over hot water and stir until smooth. Stir in the butter a little at a time, reserving a knob for later. Stir in the icing sugar, then the coffee, rum or water to form a spoonable icing. Beat in the reserved butter and spoon the icing over the éclairs. Leave to set for 1 hr before serving. This icing remains fairly soft and does not set very hard.