For the puff pastry, place the flour in a mound on a clean work surface and make a well in the centre (alternatively use a large mixing bowl). Place the butter and salt in the well and rub them together with the fingertips of one hand, gradually drawing the flour into the centre with the other hand.
When the cubes of butter have become small pieces and the dough is grainy, gradually add the iced water and mix until it is all incorporated (don’t overwork the dough – you should have a marbled effect with the butter without mixing it in properly). Bring the mixture into a ball, wrap in cling film and refrigerate for 20 minutes.
Flour a clean work surface and roll out the pastry to a 40x20cm/16x8in rectangle. (It doesn’t matter if the pastry isn’t exactly the right size - the important thing is that the corners are tidy and square.) Starting with a narrow edge of the rectangle, fold the top third down to the centre, then the bottom third up and over that.
Turn the pastry 90 degrees and again roll into a 40x20cm/16x8in rectangle. Fold it as before, then wrap the pastry in cling film and refrigerate for 30 minutes.
Remove the chilled pastry and repeat the process (rolling and folding another two times). Wrap the pastry in cling film and refrigerate for at least 30 minutes before using.
For the spring truffle pithivier, preheat the oven to 200C/400F/Gas 6.
Place the truffle in a saucepan with the jus and Madeira and poach for 10 minutes. Remove from the liquid and pat dry. Keep the liquid for the sauce.
Place the chicken in a food processor and blend. Add the egg white and blend again. Add the cream and blend to a smooth mousse. Place in a piping bag with a plain nozzle attached and set aside.
Heat a large frying pan and, once hot, add the butter. When the butter has melted, add the spinach and cook for a couple of minutes, or until the spinach has wilted. Turn out onto kitchen paper to drain.
Roll out the chilled pastry onto a floured surface and cut out eight circles using a saucer as a guide.
Take four circles of pastry and lay the spinach in a circle on top of each one – leaving a 1cm/½in gap around the edge. Place a truffle on top of each.
Pipe the chicken mousse around the truffles and place another circle of pastry on top. Using a fork, crimp the edges to seal (if desired, you can also use a knife to draw a design on the top at this stage).
Brush the top of the pithiviers with lightly beaten egg yolk and place in the oven for 10-15 minutes, or until golden-brown.
Meanwhile to make the Madeira reduction, heat a pan and add one third of the butter. When melted, add the shallot and cook for a couple of minutes, then add the Madeira and stock used to cook the truffle and cook until the volume of liquid has reduced by half.
In a separate pan, heat the remaining butter. Once hot, add the mushrooms and cook for three minutes. Set aside until ready to serve.
To serve, place a pithivier in the centre of each plate. Add the mushrooms and drizzle the reduction around the edge. Garnish with chervil.