Add the milk, butter and 125ml water to a medium saucepan over a medium heat. Heat until the butter has melted, then bring it to a rolling boil.
Remove from the heat and immediately add the flour and sugar, beating with a wooden spoon until the mixture is smooth, comes together and easily comes away from the side of the pan. Transfer to a large plate and spread out, leaving until cool enough to handle by hand. Once ready, transfer back to the pan and beat in the eggs, one by one, until smooth and glossy.
Preheat the oven to 220°C/gas mark 7. Mark two large pieces of baking paper with two rows of lines, 9cm in length and spaced evenly part. Turn them over and line two baking trays with the paper.
Using a nozzle with a width just over 1cm, fill a piping bag with the dough and pipe on to the trays, using the lines as guides. Transfer the trays towards the top of the oven and bake for 25 minutes until golden brown. Once baked, transfer to a wire rack and immediately, but carefully, slice down the side allowing any steam to escape while they cool, so they don’t get soggy. Set aside to cool completely.
Fill a small saucepan a third of the way with water and bring to a steady simmer. Set a heatproof bowl over it and add the chocolate. Let it melt, stirring occasionally, until smooth. Remove from the heat and set aside to cool.
In a heatproof medium bowl, whisk together the eggs and sugar. Add the yuzu sauce and whisk together to combine. Fill a small saucepan a third of the way with water and bring to a steady simmer. Place the egg mixture, still in the bowl, over the saucepan and reduce the heat. Stir constantly with a whisk until the mixture has thickened and easily coats the whisk. This will take around 15 minutes and will change consistency quickly, so keep your eyes on it.
Set a sieve over a large bowl and strain the mixture through to remove any bits of cooked egg. Stir in the butter while the curd is still warm. Mix until smooth. Cover with clingfilm so it’s touching the curd and place in the fridge to chill.
In a large bowl, add the cream and most of the melted white chocolate, reserving about 5 tbsp. Beat together with an electric whisk until combined and thickened. Fold in the yuzu curd until combined. Taste and if you’d like it a little tarter, add in an extra 1 tbsp of yuzu sauce.
Make the icing by combining the icing sugar with the remaining 2 tbsp of yuzu sauce, adding 1 tsp more if it seems too stiff. Fill the éclair shells with the cream filling using an icing bag and 1cm nozzle, or fill a plastic food bag and snip a bit off the corner, using that to pipe. Use a palette knife to spread the icing on to each of the eclairs. Sprinkle with the pistachios and drizzle with the remaining white chocolate. You may need to reheat slightly if it becomes too thick.