Preheat the oven to 425˚F and line two large baking sheets with parchment paper. Place the butter and water in a saucepan and bring to a steady boil until the butter is melted. Remove from the heat and add the flour, beating with a wooden spoon until a dough comes together. Place back over the heat and beat the dough in the saucepan for about 40 seconds. Remove from the heat and set aside.
Beat the eggs in a small bowl. Add the eggs little at a time, beat thoroughly with a wooden spoon until the egg is completely incorporated after each addition. You are looking to have a consistency that will hold its shape when piped. It should be smooth, shiny and just about fall from the spoon. You might not use all the egg.
Using a spatula, scoop the dough into a large piping bag fitted with a large round piping nozzle and pipe 10cm lines on the lined baking sheets, leaving about 4cm in between each line to allow for spreading. Brush each one with a little leftover beaten egg.
Place in the oven, reducing the heat to 375˚F for approximately 25 minutes until they have risen and are golden and crisp. Transfer to a wire rack and use a sharp pointed knife to pierce three holes on the underside of the buns. Allow to cool completely before filling.
For the crème pâtissière, split the vanilla pod in half and scrape out the seeds. Add this to a medium saucepan with the milk and place over a medium high heat. Bring to the boil and then turn off the heat. Scoop out the vanilla pod.
While the milk is coming to the boil, place the sugar, cornflour and egg yolks in a large mixing bowl and beat with a whisk until thick and pale.
Pour the hot milk into the bowl, whisking quickly and continuously until it is smooth and incorporated. Pour the mixture back into the saucepan and place over a medium heat beating continuously until it has thickened.
Transfer the crème pâtissière to a cold bowl, create a flat surface with the back of a spatula and wipe the top with little butter (this will prevent a skin forming). Cover directly with cling film and allow to cool completely.
For the chocolate glaze, melt the dark chocolate in a bowl set over a saucepan of hot water. When the chocolate has melted remove the bowl from the heat and gently stir through the cream. Fold in the butter until you have a shiny, spreadable chocolate glaze.
To assemble the eclairs, fill a piping bag, fitted with a small round nozzle, with the crème pâtissière and pipe into the holes on the underside of the choux buns. Spread the chocolate glaze over the top of each eclair and sprinkle with the roasted hazelnuts. Serve straight away or the eclairs will keep for 1 or 2 days in the fridge.