Combine flour, salt, and butter on a surface until the dough is grainy.
Mix in iced water until incorporated, wrap in film, and chill for 20 minutes.
Roll pastry into a 40x20cm rectangle and fold into thirds.
Turn 90 degrees, roll again to 40x20cm, fold, and chill for 30 minutes.
Repeat the rolling and folding process twice more and chill for 30 minutes.
Preheat the oven to 200°C.
Poach truffles in stock and Madeira for 10 minutes, then pat dry.
Blend chicken, egg white, and cream into a smooth mousse and place in a piping bag.
Wilt the spinach in 25g of melted butter and drain on paper towels.
Roll out pastry and cut eight large circles using a saucer as a guide.
Layer spinach and a truffle on four circles, leaving a 1cm gap at the edges.
Pipe mousse around truffles, cover with remaining circles, and crimp edges to seal.
Brush with egg yolk and bake for 10 to 15 minutes until golden.
Sauté shallots in butter, add poaching liquid, and reduce volume by half.
Sauté mushrooms in remaining butter for three minutes.
Serve pithiviers with mushrooms, Madeira reduction, and a chervil garnish.