To make the filling, pour the milk into a saucepan and add the vanilla pod and ground coffee. Bring the mixture to the boil then remove from the heat. Cover with a tight-fitting lid and leave to infuse for 1 hour.
Meanwhile, make the choux pastry. Line two baking sheets with baking paper and set aside. Pour 200 ml (7 fl oz) of water into a saucepan, add the spread and cover the pan. Heat gently until the spread has melted, then uncover the pan and bring the mixture to the boil.
Remove the pan from the heat and stir in the flour. Return to the heat and stir until the mixture forms a ball in the centre of the pan and leaves the sides clean. Remove from the heat and allow the mixture to cool for 2-3 minutes.
Whisk the whole eggs and egg white together, then beat them into the cooled pastry mixture a little at a time, preferably with a hand-held mixer, beating well between additions until the mixture is smooth and shiny and will hold a soft peak.
Spoon the choux pastry into a large piping bag fitted with a 1 cm (½ in) plain nozzle. Pipe it onto the lined baking sheets in straight lengths about 10 cm (4 in) long, spacing them 2 cm (¾ in) apart. Cut the choux pastry cleanly away from the end of the nozzle with a knife.
Freeze the piped choux pastry, uncovered, for 30 minutes, or until firm. Then cover the éclairs with foil and return them to the freezer.
Meanwhile, continue making the coffee filling: line a sieve with muslin or kitchen paper and strain the infused milk into a jug, discarding the vanilla pod and ground coffee.
Put the cornflour and sugar into a saucepan, gradually stir in the strained milk and bring it to the boil, stirring continuously. The mixture will be lumpy to start with, but continue stirring and it will thicken and become smooth as it reaches the boil. Reduce the heat to low and cook for 2-3 minutes, stirring. Then pour the filling into a bowl, cover with cling film and leave it to cool. Chill for 3 hours, or overnight, for the filling to set.
When the filling has set, heat the oven to 220°C (425°F, gas mark 7) and bake the choux pastry from frozen for 25 minutes, or until well risen, golden brown and crisp. Remove the éclairs from the oven and turn it off. Pierce each éclair at one end to allow the steam to escape, then return them to the still-warm oven for 5 minutes to dry out completely. Transfer the éclairs to wire racks to cool.
To make the icing, bring a small saucepan half full of water to simmering point. Put 4 tablespoons of cold water, the chocolate and the icing sugar into a small heatproof bowl, place it over the pan of simmering water and stir until the mixture is smooth, then set aside.
To fill the cooled éclairs, whisk the chilled coffee filling until smooth, then fold in the yoghurt. Spoon the mixture into a large piping bag fitted with a 1 cm (½ in) star nozzle.
Using a serrated knife, carefully slice the top third off each éclair. Pipe the filling generously along the base of each, then replace the tops and return the éclairs to the wire racks.
Spoon the chocolate icing along the top of each éclair, allowing it to trickle a little down the sides, and leave in a cool place until set. The éclairs can be kept, covered, in the refrigerator for 3-4 hours before serving.
Variation: if you want to leave the coffee out of the filling, flavour the milk with two vanilla pods instead.