Preheat the oven to 200 degrees Celsius / 400 degrees Fahrenheit.
Heat the water, butter, sugar and salt together in a saucepan and bring it to boiling point. Break the butter into small chunks so that the water doesn't boil before it has melted into it.
Sieve the flour and add it into the water mixture off the heat, stirring vigourously to form a smooth paste.
Return the saucepan over medium heat and continue to stir the mixture for a few minutes to cook out the flour.
Cool the mixture by turning it out into a glass bowl and put in the fridge for a few minutes, or spread it out onto a cool, clean surface.
Whisk the eggs together and add slowly into the choux pastry mixture, stirring continuously so it incorporates and forms a silky, glossy mixture that just drops off a spoon.
Pipe your profiteroles. Put the choux pastry into a piping bag and cut 1cm from the end. Pipe small mounds of the choux pastry onto a lined baking tray, leaving a gap between each one.
Flick a tablespoon of water over the choux buns to help them rise.
Bake the profiteroles in the oven for 20 minutes until they are golden brown.
Take the profiteroles out of the oven and carefully make a piping hole in the bottom rising the end of a sharp knife. Put the profiteroles back in the oven to dry out for one or two more minutes with the holes facing upwards.
Make your chocolate truffle filling. Melt the chocolate and butter together in a small bowl over a saucepan of simmering water.
Heat the cream until it reaches a steaming point.
Stir the chocolate mixture into the hot cream off the heat until thick, glossy and thoroughly incorporated. Move the mixture to a piping bag.
Pipe the profiterole filling: Pipe the chocolate profiterole filling into the profiteroles and place them into bowls.
Heat the remaining cream and when it reaches a steaming point, stir the remaining chocolate into it to form a smooth and glossy chocolate sauce.
Serve the profiteroles with chocolate truffle filling. Pour the chocolate sauce over the profiteroles and finish with more heavy cream.