Prepare the dough: In a saucepan, combine water, butter, and sugar. Heat over high heat and stir until butter melts and water starts to boil.
Add flour and form dough: Add flour all at once to the boiling mixture and stir quickly until a dough forms. Remove from heat.
Incorporate eggs: Beat the dough and gradually add eggs one at a time, beating well after each addition until smooth and falling from the mixer.
Add vanilla and salt: Add vanilla extract and a pinch of salt to the dough and beat briefly to combine.
Pipe the eclairs: Transfer dough to a plastic bag or piping bag. Cut the tip and pipe eclairs onto baking paper on a tray. Use a brush to smooth edges.
Bake the eclairs: Bake in a preheated oven at 160°C (320°F) for 15-20 minutes. Then bake on top rack for 5 minutes until lightly colored. Do not open oven during baking.
Cool eclairs: Turn off oven and leave eclairs inside for 30 minutes to cool gradually to prevent collapsing.
Prepare the filling: In a pot, combine milk, corn flour, white flour, sugar, vanilla, and egg yolks. Stir over heat until thickened and firm.
Finish filling: Remove from heat, stir in butter until melted. Cover and cool to room temperature, then refrigerate until cold.
Fill the eclairs: Cut eclairs open or slice and fill with the chilled custard filling.
Decorate: Dip the top of each filled eclair in melted milk chocolate and sprinkle crushed pistachios on top while chocolate is still wet.