To make the streusel combine flour, sugar and fleur de sel in the bowl of a food mixer. Knead with the paddle attachment then add the creamed butter.
Continue on a medium speed until the butter mixes with the dry ingredients to form a dough.
Roll out the pastry between two sheets of greaseproof paper to about 2mm thick, then place in the fridge or freezer.
When chilled use a pastry cutter to cut out 16 rounds of pastry 3cm in diameter, return to the fridge.
To make the praline cream, put the gelatine sheets in a bowl of cold water. Add the milk to a saucepan and bring to the boil.
In a mixing bowl whisk together the egg yolks and sugar until pale, then add the cornflour. Mix will to get a smooth paste.
Add half the milk to the egg mix and whisk, then pour this into the pan of remaining milk and mix again. Bring to the boil and whisk constantly.
When the cream is thickened take it off the heat and add the melted gelatine (make sure you squeeze out all of the water), the hazelnut praline paste and chilled butter.
Combine everything together then whizz with a stick blender for a smoother cream. Pour into a dish and cover with clingfilm, making sure it touches the surface of the cream to stop a skin forming.
When the cream is completely cold, put it in the bowl of a food mixer and whisk it for 3 minutes until light and use immediately.
To make the choux pastry, pour the milk and water into a saucepan then the butter cut into pieces. Bring to the boil then tip in the sifted flour, salt and sugar in one go.
Stir carefully, a dough will form immediately. Continue stirring over a medium heat for 1 minute to eliminate any moisture.
Then pour the dough into the bowl of a food mixer, fitted with the paddle attachment. Add the whole eggs one at a time, making sure they're fully mixed in before adding the next egg.
This should give you a smooth glossy choux pastry. To check if the pastry is ready, score a deep line through the surface with your finger, the line should close together very slowly.
Line two baking trays with greaseproof paper and draw a 16cm circle on each sheet of greaseproof paper. Turn the paper over and pipe eight balls of pastry 4cm in diameter on each circle. Start with the four main points of the circle and then fill in the four remaining holes. Don't worry if the buns aren't touching, they will by the time the choux is cooked.
Lay the streusel pastry on each of the choux rounds on both pastries and cook in the oven on 170C (Fan Oven) for 45 minutes. Then switch off the oven and leave to cool for 30 minutes.
When the choux is at room temperature, cut it horizontally through the middle (do this gently with a pastry knife as the choux will separate).
Use a piping bag or tablespoon to place mounds of praline cream on the bottom half of both Paris-Brest. To finish replace the top of the choux and sprinkle with icing sugar to decorate.