Preheat the oven to 200°C.
Melt the butter. Roll out the filo pastry. Brush the pastry with the melted butter then dust with cocoa powder and icing sugar. Cut the pastry into 18 rectangles and place on a lined baking sheet. Cover with a second sheet of baking parchment and then a second baking sheet. Transfer to the oven and cook for 6 minutes. Set aside.
To prepare the crème patissière: split the vanilla pod in half, scrape out the seeds and place both in a saucepan and add the milk and the chestnut cream. Bring to the boil.
Beat together the eggs and the sugar until pale and fluffy. Add the flour and the hot milk. Mix and return to the pan. Bring the mixture back to the boil and cook for a further 3 minutes, stirring continually. Pour into a bowl and cover with cling film. Set aside in a cool place for 1 hour then whisk the pastry cream until smooth. Transfer to a piping bag and set aside.
For the caramel sauce: heat a frying pan to hot and add the caster sugar. Melt, without stirring, to form a caramel. Add the salted butter and the cream. Mix well and leave to cool.
Pipe the pastry cream onto 6 of the pastry rectangles. Place another pastry rectangle on top, add another layer of pastry cream and top with a final sheet of pastry.
Serve with the caramel sauce.
«Make your caramel in a stainless steel frying pan so that you can see the colour of the caramel as it develops.»