To make the choux pastry, bring the water to the boil in a heavy-bottomed pan. Add the butter to the water to melt.
Sieve together the flour, salt and sugar onto a piece of parchment paper. When the water and butter mixture has come up to a rolling boil, add the flour mixture straight into the pan.
Beat the dough continuously with a wooden spoon as it thickens. Continue beating until the dough is smooth and comes away from the sides of the pan cleanly. Remove the dough from the pan and place in a bowl to cool.
When the dough is cool, beat the eggs in a clean bowl with a fork. Gradually add in the beaten egg mixture a bit at a time, mixing well between each addition; the mixture should now be at dropping consistency, but still firm enough to be piped in a piping bag. (You may not need to use all of the beaten egg.)
Preheat the oven to 200C/400F/Gas 6. Sprinkle a non-stick baking tray with a bit of water. (This will create steam as the choux buns cook in the oven.)
Fill a plain-nozzle piping bag with the choux dough and pipe the dough into small buns about 5cm/2in in diameter onto the baking tray, leaving at least an inch between each bun.
Bake the choux buns in the preheated oven for 20-25 minutes, until puffed, golden and crisp. Remove and cool on a cooling rack.
When they've cooled slightly, turn the buns over and pierce a hole in the base of each about the same size as the nozzle for the pastry bag and let them cool completely.
For the chocolate sauce, break the chocolate into a bowl set over a pan of gently simmering water (the water shouldn't touch the bottom of the bowl).
Add the golden syrup and the orange juice and allow the ingredients to melt together to form the sauce, stirring gently.
For the pistachio cream, using a hand whisk, whip the cream until it forms soft peaks.
Chop the pistachios and gently fold into the whipped cream. Gently stir in the icing sugar with a metal spoon.
Fill a clean piping bag with the pistachio cream and use to fill the cooled profiteroles.
To serve, arrange the profiteroles on a plate and drizzle the chocolate sauce over the top.