Preheat the oven to 200°C, fan 180°C, gas 6.
Sieve the flour onto a small piece of baking parchment. Gently heat the butter in a small pan with 150ml cold water, until the butter has melted, then bring to the boil. Once the mixture reaches boiling point, remove immediately from the heat and tip in the flour all at once. Beat with a wooden spoon until the mixture forms a smooth ball, then leave to cool completely.
Using an electric hand mixer, beat the eggs into the mixture a little at a time, mixing thoroughly between each addition, until you get a smooth, glossy paste.
Spoon into a piping bag fitted with a plain 1.5cm nozzle. Pipe 8 éclairs onto a non-stick baking sheet, leaving a gap between each one. Bake for 10 minutes, then increase the heat to 220°C, fan 200°C, gas 7 and bake for a further 15 minutes. Remove and cool on a wire rack. Using a sharp knife, carefully make a small cut in the side of each éclair to allow the steam to escape.
To make the filling, whisk the cream until soft peaks form. Sift in the icing sugar into the bowl and fold through. Spoon into a piping bag fitted with a 1cm nozzle and fill each éclair using the opening made earlier.
Melt the chocolate and butter together in a small bowl in the microwave or in a heatproof bowl over a pan of simmering hot water. Leave to cool slightly, until it forms a thick, coating consistency, and then spoon over the éclairs and leave to set.