This is a gluten-free version of the much-loved afternoon tea classic. A French star tip is not essential, but it does create elegant ridges down the buns as they bake –you can use a plain 1cm nozzle, if you prefer.
Heat the oven to 200°C/180°C fan/400°F/Gas 6.
Sift the flour onto a sheet of baking paper. Fold the edges of the paper to create a shoot.
Put 320ml of water, and the butter, salt and sugar in a saucepan over low heat, stirring from time to time until the salt and sugar dissolve and the butter melts. Turn the heat up to high and bring the liquid to a rolling boil. As soon as the butter gathers in the centre, remove the pan from the heat and tip in the sifted flour.
Using a wooden spoon, vigorously beat the mixture to form a thick paste with no dry bits. Cook the pastry over medium heat, still stirring vigorously, until the dough comes away from the sides of the pan to form a ball (about 20–30 seconds).
Remove the pan from the heat and set the dough aside to cool for 10 minutes. (If you add the eggs when the dough is too hot, they will cook.)
Beat one third of the eggs into the warm choux paste, until fully combined. Beat in another one third of the egg, then add the remaining egg in small increments until the choux paste is smooth and shiny and drops reluctantly from the spoon (you may not need all the egg). Stir in the baking powder. To test if the dough is the right consistency, make a shallow trough it (with a spoon or your finger) – the sides should stay upright for a few seconds, then slowly slump back into the middle.
Fill the piping bag fitted with the French tip (or plain) nozzle with the choux paste. Draw 16–18 10cm-long lines, each at a 45°C angle, on the baking paper lining the baking sheets, spacing the lines 5cm apart. Turn over the baking paper so that the drawn lines are on the underside.
Using the lines as a guide, pipe 16–18 lines of choux dough, then dust the tops of the piped dough with icing sugar. Place the baking trays in the oven and bake the éclair buns for 35 minutes, then briefly open the oven door to let the steam escape, close it, and bake for a further 5–10 minutes, until the éclair buns are crisp and golden. Remove from the oven.
Make 3 small holes in the bottom of each éclair bun (a small piping nozzle will help with this). Return the éclair buns to the oven, bottom side up for a further 3–4 minutes, then transfer the buns to a wire rack and set them aside to cool completely.
When you’re ready to assemble the éclairs, make the chocolate glaze. Put the chocolate in a heatproof bowl. Heat the cream, butter and golden syrup in a small saucepan over a low heat until it’s just beginning to simmer (don’t let it boil). Pour the cream over the chocolate and leave it to stand for 2 minutes, then stir gently with a silicone spatula until the glaze is smooth and glossy. Set aside.
Make the Chantilly cream. Combine the cream, icing sugar and vanilla in a chilled bowl and, using an electric hand whisk, whip it to soft peaks. Transfer the cream to the piping bag fitted with the plain nozzle.
Using the holes you made in the bottom of the buns, fill the cooled éclairs with the Chantilly cream. Then, dip the top of each éclair in the chocolate glaze, gently smoothing the top with a small palette knife and so that the glaze comes about one third to one half of the way down the éclair. If it runs too freely, leave the glaze to cool and thicken for a while longer and try again. Once all the éclairs are decorated, serve immediately.