First, make the craquelin. Combine the butter, sugar, flour and hazelnuts in a bowl, then spread out over a sheet of baking parchment. Cover with another sheet of parchment and roll the mixture out until it's 2mm thick. Put in the freezer to chill while you prepare the choux.

While the craquelin chills, sift the flour onto a sheet of baking parchment and fold in half (you will use the parchment as a funnel for adding the flour). Pour the milk, sugar, 1/2 tsp salt and the butter into a pan with 150ml water. Bring to the boil, then remove from the heat and quickly tip in the flour using the parchment as a funnel. Quickly and vigorously stir in the flour using a whisk. Return the pan to a low heat and, using a wooden spoon, stir for 1 min, beating well. The dough should start to come away from the bottom of the pan easily. Transfer the mixture to a stand mixer and beat for 30 seconds to cool slightly (or do this with an electric whisk in a bowl). Slowly pour in the egg, beating continuously using the mixer or whisk while checking the consistency of the dough – it should be smooth and silky, and you may not need all the egg. When ready, the dough should be able to slowly drop off a wooden spoon. (You can also mix in the egg by hand using a wooden spoon.)

Heat the oven to 180C/160C fan/gas 4. Spoon the choux into a piping bag fitted with a large round nozzle. Draw 14-16 circles over two sheets of baking parchment using an 8cm round biscuit cutter as a guide, then flip over and use to line two large baking trays (you should get at least 14 mini Paris-Brest, but may get up to 16, depending on your mixture). Pipe rings of choux inside the circle templates, leaving the middles clear. Stamp out 14-16 circles of craquelin using the same 8cm cutter, then use a 5cm cutter to stamp out the middles to make rings. Put these on top of the piped choux, then bake for 30-35 mins until risen, golden and completely dry – they should sound hollow when gently tapped, and feel light. Remove from the oven and leave to cool completely on the baking trays.

For the mousseline, pour the milk into a pan and warm over a low heat, swirling the pan to ensure it doesn’t catch, until it just starts to steam. Combine the egg yolks, sugar and cornflour in a heatproof bowl using a whisk. Slowly whisk in the hot milk, then pour back into the pan and return to a medium heat. Continue to whisk until the mixture thickens, then whisk in the hazelnut butter and a large pinch of salt. Remove from the heat and spread the mixture out over a plate to cool completely. Once cooled, tip the butter into a bowl and beat using an electric whisk until smooth and creamy. Add half the cooled hazelnut mixture and beat well, then beat in the rest until smooth and just holding its shape.

To assemble the individual Paris-Brests, cut the choux pastry rings in half through the equator using a serrated knife. Spoon the hazelnut mousseline into a piping bag fitted with a small round or fluted nozzle, and pipe into the bottom half of the choux. Sandwich with the top half and generously dust with icing sugar.