Sift the flour with the salt on to a sheet of greaseproof paper. Put the butter into a medium heavy-based pan with 150ml water. Heat gently until the butter melts, then bring to a rapid boil. Take off the heat and immediately tip in all the flour and beat throughly with a wooden spoon until the mixture is smooth and forms a ball. turn into a bowl and leave to cool for about 10 minutes.
Gradually add the eggs to the mixture, beating well after each addition. Ensure that the mixture becomes thick and shiny before adding any more egg – if it’s added too quickly, the choux paste will become runny and the cooked buns will be flat.
Sprinkle a large baking sheet with a little water. Using two damp teaspoons, spoon about 18 small mounds of choux paste onto the baking sheet, spacing well apart to allow room for them to expand. Alternatively, spoon the choux paste into a piping bag fitted with a 1cm plain nozzle and pipe mounds on to the baking sheet.
Bake at 220oc for about 25 minutes or until well risen, crisp and golden brown. Make a small hole in the side of each bun to allow the steam to escape and then return to the oven for a further 5 minutes or until thoroughly dried out. Slide on to a large wire rack and set aside to cool.
To make the sauce, put the chocolate and cream into a medium pan with 4 tbsp water. Heat gently, stirring occasionally, until the chocolate melts to a smooth sauce; do not boil. Remove from the heat.
To assemble, lightly whip the cream with the vanilla extract and sugar until it just holds its shape. Pipe into the hole in each choux bun, or split the buns opens and spoon in the cream. Chill for up to 2 hours.
Just before serving, gently reheat the chocolate sauce. Add Grand Marnier and caster sugar to taste, if you like. Divide the choux buns among serving bowls and pour over the warm chocolate sauce. Serve immediately.