Preheat the oven to 210°C/425°F/Gas mark 7, and grease two baking trays.
Combine the butter and 225ml/8fl oz water in a large heavy saucepan. Stir over a medium heat until the butter melts. Increase the heat, bring to the boil, then remove the pan from the heat.
Add the flour to the saucepan all at once, and quickly beat into the buttery water using a wooden spoon. Return to the heat and continue beating until the mixture leaves the sides of the pan and forms a ball. Transfer to a large bowl and cool slightly.
Beat the mixture to release any remaining heat. Gradually add the egg, about 3 teaspoonfuls at a time. Beat well after each addition until all the egg has been added and the mixture is glossy. Spoon the mixture into a piping bag fitted with a 1.5cm/5⁄8in plain nozzle.
Sprinkle the baking trays lightly with water. Pipe 15cm/6in lengths onto the trays, leaving room for them to expand. Bake for 10–15 minutes. Reduce the oven temperature to 180°C/350°F/Gas mark 4, and bake for a further 15 minutes. Cool on a wire rack.
Split each cooled éclair and remove any uncooked dough. Fill the centre with cream.
Melt the chocolate in a bowl set over a saucepan of simmering water. Spread the melted chocolate over to the top of each éclair, in a line along the top. Allow to set, then serve.