For the rough puff pastry, sift the flour and salt into a large bowl. Rub the butter into the flour using your fingers until you have small lumps of butter remaining (stop before it starts to look like breadcrumbs as you want squashed pieces of butter showing in the mix). Add 150ml/5fl oz water, or enough to bring the mix together (you may not need all the water).
Flour the work surface and roll out the dough to a neat rectangle that is about three times longer than its width. Fold in one third of dough to the centre and cover this with the remaining third. Cover and leave to rest in fridge for 20 minutes. Roll the pastry out again to the previous length and width, and fold in thirds as before. Cover and let rest again for at least 20 minutes in fridge (or if you have time an hour).
Preheat the oven to 180C/350F/Gas 4 and line a baking tray with parchment paper.
For the choux pastry, in a medium-sized pan over a low heat, bring the milk to a simmer. Add the butter, 124ml/4fl oz water, salt and sugar and bring to boiling point. Add the flour and mix with a wooden spoon. Cook for about a minute until the paste leaves the sides of the pan. Transfer to the bowl of an electric mixer and set aside to let it cool slightly before adding the eggs one at a time, beating constantly. The paste will become smooth and shiny.
To make the small choux buns, using a piping bag and plain nozzle pipe 16 small choux buns on to the lined baking tray. Set aside the remaining choux pastry. Brush each choux bun with egg wash and bake on the middle shelf for 15-20 minutes. Turn off the oven and leave the choux buns in the residual heat for 10 minutes to dry out. Remove the buns from the oven and then return the oven to 180C/350F/Gas 4.
Cut a 27cm/10in disk from the rough puff pastry and place it on a lined baking tray. Pipe a spiral of choux paste starting from centre of the disk, stopping when two thirds of the disk is covered. Now pipe a border of choux paste round the edge leaving a 5mm/¼in of space at the outer edge of the pastry base (the choux ring will expand to reach it). Brush with egg wash and bake at 180C/350F/Gas 4 for 25–35 minutes. Turn off the oven and leave in the residual heat for 10 minutes to allow the choux pastry to crisp up. Transfer to a cooling rack.
For the crème pâtissière, whisk the egg yolks and a third of the sugar in a bowl until light in colour and fluffy. Add the flour and mix thoroughly. In a large pan, heat the milk, vanilla paste and the rest of the sugar. When it comes to the boil, pour it over the egg yolk mixture and whisk together. Transfer to a clean saucepan, bring it back to the boil - whisking continuously - and cook for a few minutes until thick. Pass through a sieve into a bowl and put some cling film directly onto the surface to prevent a skin forming. Transfer to the fridge to chill.
When the crème pâtissière has cooled, spoon it into a piping bag with a medium circular nozzle attached and pipe it into the small choux and over the pastry base.
For the crème Chantilly, melt the chocolate in the microwave and then set aside to cool. In a chilled bowl combine both creams with the icing sugar and vanilla extract. Whisk until it thickens enough to pipe, but do not overbeat (it should be a light consistency). Transfer half the crème to another bowl and add the cooled, melted chocolate to it. Fold in carefully (do not whisk) and then spoon into a piping bag with a large round nozzle attached.
In another piping bag with the same nozzle attached, add the plain crème Chantilly. Pipe alternating rows of both creams to cover the crème pâtissière completely.
Make a caramel by melting the sugar and 50ml/2fl oz water in a heavy-based pan. Heat until the caramel is a deep amber colour, then remove from the heat and plunge the base of the pan into cold water to prevent the remaining caramel burning.
Line a baking tray with parchment paper. Carefully dip each choux into the caramel and transfer to the baking tray, caramel-side down. When the caramel has cooled and set, attach the choux buns round the outside of the choux ring using the caramel (heat it a little to soften if you need to). Leave a 1½cm/½in space between each choux.
To decorate, make chocolate curls by scraping a vegetable peeler down the edge of the block of chocolate. Pipe crème Chantilly between each choux and place a chocolate curl on top.