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Preheat the oven to 200°C/fan oven 180°C/Gas Mark 6. Lightly grease 2 baking trays with a little TREX. Sift the flour and salt onto a plate.
Put the water and TREX into a saucepan. Heat gently, so that the TREX melts, then bring up to the boil. Remove from the heat, then immediately add all the flour, beating vigorously with a wooden spoon so that the mixture forms a soft ball. Cook over a low heat for about 30 seconds. Cool for 5 minutes.
Gradually add the beaten eggs to the flour mixture using either a hand-held electric mixer or a wooden spoon, beating well between each addition. The mixture should be very smooth, shiny and thick.
Spoon the mixture into a large piping bag fitted with a plain nozzle. For eclairs, pipe twelve 10cm (4in) lengths on the baking sheets, allowing room for them to spread and rise. For profiteroles, pipe about 30 balls instead.
Bake for 10 minutes, and then open the oven door slightly for just 2 seconds to allow the steam to escape. Bake for a further 5-10 minutes until the pastry is golden brown and crisp.
Remove from the oven and split each éclair lengthways or stab each profiterole with a sharp knife to let the steam escape. Transfer to a wire rack to cool completely.
Fill the eclairs or profiteroles with whipped cream. Melt the chocolate in a bowl placed over a saucepan of simmering water. Dip the tops of the eclairs or profiteroles in the chocolate or spread it over them with a palette knife. Keep chilled.