Preheat the oven to 190C/375F/ Gas 5. To make the choux pastry, put 200ml water into a saucepan and add the butter. Heat until the butter has melted, then add the flour. Stir with a wooden spoon until the flour is evenly incorporated, and the mixture comes away from the side of the pan. Remove from the heat and leave to cool a little. Beat in the eggs, until the mixture is smooth and glossy.
Draw a circular template (around the size of a dinner plate) onto a sheet of baking parchment and lay this on a baking tray. Spoon the choux mixture into the piping bag and pipe adjoining circles (or straight lines) all around the outer edge of the template. Cook for around 25-30 minutes, until golden and risen (it helps to throw a few ice cubes into the bottom of the oven to create steam, which will help the pastry rise). Remove from the oven and transfer to a cooling tray.
In the meantime, put the raisins into a bowl, pour over the Pedro Ximénez and leave to macerate.
Put the nuts on a baking tray lined with baking parchment and roast for around 5 minutes, until golden. Put the caster sugar into a heavy-based pan and heat gently until the sugar starts to melt. Turn up the heat and bubble until the mixture turns a medium amber gold. Quickly tip the nuts into the pan and turn the whole lot back out onto the parchment that the nuts were roasted on. Spread the mixture into an even layer and leave to cool.
For the praline, put half the cooled nuts into a food processor and blitz to a textured powder. Break the remaining nuts into shards and set aside.
Pour the cream into a large bowl. Drain the raisins, then add the Pedro Ximénez that they have been soaking in to the bowl with the cream. Add the icing sugar and whisk until the mixture forms fairly stiff peaks. Fold in the soaked, drained raisins and powdered nuts. Transfer the mixture to the piping bag with a nozzle big enough that the raisins don’t get stuck. Cut the Paris- Brest in half horizontally and fill with the praline cream. Pop the top back on.
Heat the 50g caster sugar to make a caramel and drizzle over the top of the dessert. Decorate with the caramel nut shards, using any last bits of caramel in the pan to pull into spun sugar to decorate.