Preheat oven to 190ºC bake and line two large oven trays with baking paper.
Combine water, butter, sugar and salt in a heavy-based saucepan. Cook over medium heat, stirring occasionally.
Meanwhile, sift flour onto a piece of baking paper. When water starts to boil (butter should be melted by this point), remove from the heat and add flour.
Mix vigorously until well blended. Return to low heat and continue to mix until the dough leaves the sides of the saucepan and forms a film or oil droplets on the bottom of the saucepan (about 2 minutes).
Transfer to the bowl of a stand mixer or other large bowl and leave to cool for 5-10 minutes.
Whisk eggs together in a jug. Add vanilla essence to pastry dough, then add eggs a little at a time, beating well after each addition (use a stand mixer or hand-held electric mixer) until thoroughly incorporated. Do not add all the egg unless it is needed. The dough is right when it has a glossy sheen and a pipeable consistency. When creating a trough in the surface of the dough with your finger, the sides should stay upright and not collapse into the dough. The mixture should also hold its shape on a spoon.
Transfer the dough to a large piping bag fitted with a large open star tip. Pipe dough into 7cm strips on prepared trays (for more consistent results, pipe and bake one tray at a time). Use a damp finger to flatten any pointy ends. Bake for 25 minutes, until pastry, puffs up and is golden brown all over. Remove from oven and prick each eclair a couple of times with a toothpick to release the steam. Transfer to a cooling rack and leave in a draft-free place to cool completely before assembling.
Whisk cream and icing sugar together until softly whipped. Transfer to a large piping bag fitted with a large open star tip. Cut into the sides of the eclairs and pipe cream inside.
Place chocolate and cream in a microwave-safe bowl and microwave on low in 30 second bursts, stirring in between, until melted and smooth. Chill in fridge for 5 minutes, then spread over the top of the eclairs and leave to set.
Fill cooled pastry shells immediately or store in an airtight container for up to 2 days. Pastry shells can also be frozen in an airtight container. They can be re-crisped by baking at 180ºC fan bake for 3 minutes.
These eclairs are also delicious as a raspberry and white chocolate version. Place fresh raspberries in the eclairs with the cream and use white chocolate in the topping. Sprinkle topping with freeze dried raspberries.