Heat the butter with 200 ml water in a saucepan over a medium heat.
Bring to the boil, then remove from heat (the butter should be melted)
Add the flour all at once and stir well with a wooden spoon until the mixture comes together to form a ball.
Allow to cool slightly, then add the beaten egg a little at a time.
The mixture should be smooth and have dropping consistency (you may not need to add all the eggs)
Spoon the mixture into a piping bag with a large plain nozzle.
Pipe 10cm pieces of pastry onto a greased baking tray, leaving space for spreading.
Bake in a preheated oven for 30 minutes, then switch off and leave the eclairs in the oven for a further 10 minutes.
Cut open the eclairs and leave to cool.
To make the ganache,heat the cream in a saucepan.
Just before it boils, remove from the heat and add the white chocolate and vanilla.
Stir until smooth.
Set aside to cool, then refrigerate for at least two hours before beating with an electric beater until thick.
Stir in half the pistachios.
Spoon the filling into the eclairs and drizzle with dark chocolate.
Sprinkle with the remaining pistachios and refrigerate until ready to serve.