Preparing the eclairs: In a saucepan, melt the butter with the water, sugar and the pinch of salt. Add the sieved flour, dry the paste gently over the heat: it must come away from the edges of the saucepan. Away from the heat, add the eggs one at a time, while stirring. Using a piping bag fitted with a straight 2 cm nozzle, pipe out strips about 6-8 cm long onto a baking sheet, ensuring that they are well separated from each other. Brush these eclairs with egg yolk, and cook in the oven at 180°C for around 20-25 minutes. Then reduce the heat to 150°C and continue cooking for around 10 minutes. Remove the eclairs from the oven when they are golden on the outside (the inside must remain soft). Leave to cool on a rack.
Preparing the cream : In a saucepan, heat the milk with the coffee. In a mixing bowl, beat the egg yolks with the sugar until pale and creamy. Add the flour and cornflour. Pour the boiling milk onto this mixture and mix well. Put the mixture back into the saucepan and bring to the boil, stirring constantly. Leave to cook for a few minutes and remove from the heat. Leave to cool.
Preparing the eclairs : Using a small piping bag, make a hole in each end of each eclair. Fill the eclairs with the cream. Warm the fondant icing and colour it with the coffee extract. Ice the eclairs.