Preheat the oven to 200C/390F/Gas 6 and grease a large baking tray with butter. Sift the flour and salt twice.
Put the water and butter into saucepan, gradually heat until the butter melts and then increase the temperature. As soon as the water reaches a brisk boil, lower the heat again and tip in all of the flour. Beat quickly until the mixture forms a ball and leaves the sides of the pan clean.
Remove the pan from the heat and cool slightly. Add the eggs, one at a time, beating hard until the mixture is shiny and firm.
Spoon 20 equal balls of the mixture onto the buttered baking tray (alternatively you can pipe the pastry onto the tray for a neater finish).
Bake in the oven for 20-25 minutes, or until golden-brown and puffed up. Transfer to a wire rack to cool.
For the crème patissière, whisk the egg yolks and sugar together in a bowl until pale and light. Fold in the cornflour and flour.
Bring the milk to the boil in a small saucepan and then whisk it gradually into the egg mixture.
Pour the mixture into a clean pan and bring it slowly to the boil until it thickens. Simmer for a minute and then take the pan off the heat.
Cover with cling-film to prevent a skin from forming.
When the profiteroles are cooled, slice each one in half with a sharp knife. Fill with the crème patissière.
For the warm chocolate sauce, heat the chocolate and cream in a heatproof bowl placed over a pan of simmering water until melted - do not allow the water to touch the bottom of the bowl. Stir to combine.
Serve the warm chocolate sauce drizzled over the filled profiteroles.