Preheat the oven to 220°C (200°C fan bake). Line 2 baking trays with baking paper.
To make the éclairs, place the water, milk, butter, sugar and salt in a large saucepan and bring to the boil. Remove from the heat, add all the flour at once and stir vigorously to combine.
Place back over a medium heat for 1–2 minutes, stirring constantly to release the extra moisture.
Transfer to a large mixing bowl and beat for about 1 minute to cool slightly.
Add the eggs one at a time, beating well after each addition. Beat until the dough is smooth and forms a thick ribbon when pulled up.
Transfer to a piping bag fitted with a 13mm round piping tip. Pipe about 18 éclairs, each about 10cm long and 2cm wide, onto the prepared trays, leaving 4cm in between them.
Bake for 10 minutes. Without opening the oven, reduce the temperature to 160°C (140°C fan bake) and bake for a further 45 minutes or until they are golden brown and sound hollow when you tap the base. Transfer to a wire rack and leave to cool.
To make the Coffee Cream, dissolve the instant coffee in the boiling water. Cool.
Whip the cream and icing sugar in a medium bowl until stiff peaks form. Fold in the cooled coffee and half the hazelnuts. Refrigerate until ready to serve.
When ready to serve, cut the éclairs in half lengthwise and fill with Coffee Cream.
To make the Chocolate Topping, break the chocolate into a microwave-safe dish, add the oil and microwave for 1 minute. Stir, then microwave in 20-second bursts, stirring between bursts, until melted and smooth.
Spread the Chocolate Topping over the éclairs and sprinkle with the remaining hazelnuts.
A Table at Tangleby
In this exceptional book, it shares more than 170 delicious dishes, from simple snacks to dinner parties that are guaranteed to impress your guests.