Begin by making the choux pastry, put the butter, salt, sugar and 120ml water in a medium pan over a medium-high heat. Once the butter has melted and the mixture is at a rolling boil, add the flour and quickly stir together with a wooden spoon until the mixture forms a dough.
With the pan still on a low heat, stir vigorously for 2 minutes, then tip the dough into a bowl and beat for a few minutes until it stops steaming. These two actions help to cook the flour and dry out the dough, which in turn helps it to absorb more egg. This helps the choux pastry to expand properly as it bakes.
Add the eggs, one at a time, beating until fully absorbed before adding the next. Depending on the flour used and how much water evaporated as you made the dough, the choux pastry will need varying amounts of egg, so the above is given as a guide.
With this recipe I usually add two eggs and then very slowly start adding the remaining egg, checking the texture of the dough after each addition. You are looking for a dough that has a shine and when it is lifted from the bowl, it should fall from the spatula in a ribbon that forms a "V" shape. If the dough doesn't contain enough egg, it won't expand properly and will be prone to cracking as it bakes; if there is too much egg, the dough won't hold its shape and will collapse as it bakes.
To prevent the dough from drying out and forming a skin, immediately put the dough into a piping bag with a 1.5cm plain round piping tip fitted.
Take the choux pastry and pipe it into silicone hemipshere moulds, You will need two of these.
Place them in the freezer for at least a couple of hours to set.
Next make the hazelnut praline mousseline, place the milk in a saucepan along with the vanilla bean paste and bring to the boil.
In a large bowl combine the eggs, egg yolks, cornflour and sugar in a bowl and whisk until pale yellow. Slowly add the boiled milk whisking the whole time.
Place the mixture back in the saucepan and whisk over the heat until it thickens, remove from the heat and and add 125g butter until melted and combined. Cover the bowl with clingfilm and place in the fridge until it's cold.
Next make the craquelin, mix the flour, sugar and butter together in a bowl, using your fingertips rub the ingredients together until they resemble breadcrumbs.
Then press the breadcrumbs together until they form a dough. Place the dough between two sheets of greaseproof paper and roll out the dough until it's 2mm thick. Place the dough on a baking tray, still covered in the greaseproof paper and put it in the freezer.
After about 30 minutes, preheat the oven to 180C (160C Fan) Gas 4 and line two baking trays with baking parchment, remove the craquelin from the freezer and cut out rings that are the same size as the base of the choux buns. Take the frozen choux out of the freezer and place it on the baking trays with spaces between to allow them to expand. Top each choux bun with the craquelin and place in the oven to bake from frozen for 45 minutes or until golden brown and puffed up. Switch off the oven and leave in the oven for a further 30 minutes.
Take the choux buns out of the oven and allow them to cool.
Remove the creme patissiere from the fridge and give it a stir, place the remaining butter in the bowl of a stand mixer fitted with the whisk attachment, whisk the butter to beat it and smooth it. Add the creme patissiere in a few batches making sure it's mixed in fully between each addition. Finally add the hazelnut praline paste and give it another mix. Place the hazelnut praline mousseline in a piping bag fitted with a french tip and place in the fridge to chill a little. Don't leave it in the fridge for too long as the butter in the mousseline will make it set too hard.
Take the cooled choux buns and place them on a wire rack or plate, using a serrated knife slice each choux bun in half.
Take the hazelnut praline mousseline and pipe a generous amount on each choux bun and sandwich them together - there will be plenty of filling to go round, so don't skimp on the filling. Top each choux pastry with the corresponding half and place the icing sugar in a sieve and dust the top of the choux bun.