Make the choux pastry: Place the water, butter, sugar, and salt in a medium saucepan over a medium heat. Bring to a gentle boil — you'll see the butter melt completely and the liquid bubbling steadily. Tip in the sifted flour all at once and beat vigorously with a wooden spoon. The mixture will form a smooth, thick paste that pulls away from the sides of the pan. Cook for another minute, stirring constantly, until you see a thin film form on the bottom of the pan — that's the starch cooking out.

Cool the paste slightly: Tip the paste into a mixing bowl and let it sit for 5 minutes. You want it warm, not hot, so the eggs don't scramble. If you touch the side of the bowl, it should feel like a warm bath.

Add the eggs: Pour in the beaten eggs a little at a time, mixing thoroughly after each addition. The mixture will look glossy and smooth, and when you lift the spoon, the dough should fall in a thick ribbon that holds a V-shape for a second. If it's too stiff, add a teaspoon of water. If it's too runny, you've added too many eggs — but you'll know because the dough will be slack and won't hold its shape.

Pipe the eclairs: Preheat the oven to 200°C (fan 180°C). Line two baking trays with baking paper. Spoon the choux pastry into a piping bag fitted with a 1cm star nozzle. Pipe 12cm lengths onto the trays, leaving 5cm between each — they expand dramatically. Smooth any peaks with a wet finger; otherwise they'll burn.

Bake the choux: Bake for 20 minutes, then reduce the oven temperature to 180°C (fan 160°C) and bake for another 10 minutes. Do not open the door during the first 20 minutes — the steam needs to stay trapped. You'll know they're done when they are deep golden brown, feel light as a feather, and sound hollow when tapped on the bottom. Let them cool completely on a wire rack.

Make the custard: While the choux bakes, whisk the egg yolks, sugar, and cornflour in a heatproof bowl until pale and thick — it should look like a smooth, creamy paste. Heat the milk in a saucepan until it's steaming and small bubbles appear around the edge. Pour a third of the hot milk into the egg mixture, whisking constantly, then pour everything back into the saucepan. Cook over a medium heat, whisking continuously, until the custard thickens to a pudding-like consistency and you see the first large bubble pop on the surface. Remove from the heat, stir in the vanilla, and cover with cling film touching the surface to prevent a skin forming. Chill for at least 30 minutes.

Assemble the eclairs: Once the choux buns are cool, use a skewer to poke a small hole in the base of each one. Spoon the chilled custard into a piping bag fitted with a small round nozzle, insert the nozzle into the hole, and fill each eclair until you feel it swell slightly. You'll see a tiny bit of custard peek out of the hole when it's full.

Make the glaze: Break the dark chocolate into small pieces and place in a heatproof bowl with the double cream and golden syrup. Set the bowl over a pan of barely simmering water — the bowl should not touch the water. Stir gently until the chocolate is melted and the glaze is smooth and glossy, the colour of dark treacle. Remove from the heat.

Glaze and serve: Dip the top of each filled eclair into the warm glaze, letting the excess drip off. Place on a wire rack to set for 5 minutes. The glaze will become firm but still slightly soft to the touch. Serve immediately or chill for up to 2 hours.