For the crispy-skinned chicken, remove the bone from each chicken thigh and pierce the skin with a knife. Heat the stock in a deep frying pan. Add the thighs skin-side down and cover with a cartouche (a round of greaseproof paper with a hole in the middle to allow steam to escape). Poach the thighs for five minutes then remove the chicken from the pan, put on a plate and place another plate on top to weigh them down. Allow to cool in the refrigerator while you make the gnocchi and sauce. Reserve the vegetable stock to use in the mushroom sauce.
To make the gnocchi, cook the potatoes in a pan of as little water as possible until tender. Drain and mash or rice the potatoes while they are still warm.
Turn out the mash onto a work surface, season with salt and pepper and add the herbs. Slowly add the flour in batches, kneading gently by folding the edges of the dough into the center and pressing down lightly before sprinkling in each further addition of flour, until the dough is soft. Don’t overwork the dough or you will spoil the structure and the gnocchi will be powdery on the outside and pulpy on the inside.
Roll the dough into a Frankfurter-sized sausage. Cut the sausage into 3cm/1¼in pieces.
Press each piece of dough against the back of a fork so that it curves slightly and the prongs leave indentations (which will help the gnocchi hold the sauce).
Bring a large pan of salted water to the boil. Add the olive oil and drop in the gnocchi. Cook for a minute, or until the gnocchi rises to the surface. Remove from the pan, plunge into a bowl of ice-cold water and drain.
For the mushroom sauce, heat the butter and olive together in a frying pan until melted. Quickly cook the shallot and garlic until just softened. Add all the mushrooms to the pan along with some of the reserved poaching liquid. Cook for five minutes, then remove from the heat and stir in the crème fraiche. Transfer to a food processor and blend until smooth. Check the seasoning and add salt and pepper as required. Set aside until ready to serve.
Just before serving, cut eight squares of greaseproof paper (large enough to cover the chicken thighs). Sprinkle each piece of paper with cep powder. Put each piece of chicken skin-side down on top of a square of greaseproof paper. Season with salt and pepper, then top with a second piece of greaseproof paper and fold the edges of the two squares of paper to make a parcel. Place each chicken parcel into a frying pan and with a weight on top. Place over a medium heat until the chicken skin crisps up and the meat is cooked through. Remove from the paper and roll the chicken in the chopped parsley.
To serve, return the sauce to the pan to heat through and stir through the drained gnocchi. Cut the chicken into slices and serve with the gnocchi and sauce.