Make choux pastry: Preheat oven to 390 °F (200 °C). In a saucepan, combine water, milk, butter, salt, and sugar. Bring to a boil until butter melts. Remove from heat, stir in flour all at once, then return to low heat and stir for 2‑3 minutes until mixture pulls away from pan and forms a dough.

Transfer dough to mixer or bowl and let cool briefly. Beat in eggs one at a time until the dough is smooth, glossy, and drops slowly off the spoon.

Pipe 10–12 oblong éclairs (~4 inch) onto parchment‑lined baking sheets. Bake at 390 °F for 3–4 minutes, reduce heat to 180 °C (≈ 350 °F), and bake 12–15 minutes until golden and firm. Pierce shells to release steam and let cool completely.

Prepare pistachio pastry cream: In a saucepan, heat milk with pistachio paste until steaming. Whisk egg yolks, sugar, cornstarch, and salt; temper with hot milk slowly. Return to saucepan and whisk over medium heat until mixture thickens and boils. Remove from heat, stir in white chocolate until melted, then whisk in vanilla. Chill covered until fully cool.

Fill éclairs: Using a small round tip, pipe pistachio cream into each cooled éclair shell from one or both ends until filled.

Make ganache: Warm heavy cream, pour over chopped dark chocolate, let sit 1 min, then stir until smooth. Dip tops of filled éclairs into ganache and allow excess to drip off. Immediately sprinkle with chopped pistachios.

Serve immediately or refrigerate in airtight container for up to 3‒4 days; best consumed within 1 day while pastry remains crisp.