Preheat the oven to 200C/400F/Gas 6. Place a small roasting tin in the bottom of the oven to heat.
For the choux pastry, place the water, sugar and butter into a large saucepan. Heat gently until the butter has melted.
Turn up the heat, then quickly pour in the flour and salt all in one go.
Remove from the heat and beat the mixture vigorously until a smooth paste is formed. Once the mixture comes away from the side of the pan, transfer to a large bowl and leave to cool for 10-15 minutes.
Beat in the eggs, a little at a time, until the mixture is smooth and glossy and has a soft dropping consistency - you may not need it all.
Lightly grease a large baking sheet. Using a piping bag and plain 1cm/½in nozzle, pipe the mixture into small balls in lines across the baking sheet.
Place the baking sheet into the oven. Before closing the oven door, pour half a cup of water into the roasting tin at the bottom of the oven, then quickly shut the door. This helps to create more steam in the oven and make the pastry rise better. Bake for 25-30 minutes, or until golden-brown - if the profiteroles are too pale they will become soggy when cool.
Remove from the oven and turn the oven off. Prick the base of each profiterole with a skewer. Place back onto the baking sheet with the hole in the base facing upwards and return to the oven for five minutes. The warm air from the oven helps to dry out the middle of the profiteroles.
For the filling, lightly whip the cream with the orange zest until soft peaks form. When the profiteroles are cold, use a piping bag to pipe the cream into the profiteroles.
For the chocolate sauce, place the water and sugar into a small saucepan and bring to the boil to make a syrup. Reduce the heat to a simmer and place the chocolate into a heatproof bowl set over the pan. Heat, stirring occasionally, until melted. Take the pan off the heat, pour the syrup mixture into the chocolate and stir until smooth and well combined.
To serve, place the stuffed profiteroles into a large serving dish and pour over the chocolate sauce. Serve hot or cold with single cream.