Preheat the oven to 180C/350F/Gas 4.
Place the potatoes on a baking tray and bake them for one hour, or until completely cooked through.
Remove the potatoes from the oven and set aside to cool.
Once cool, cut the potatoes in half and scoop 200g/7oz of the flesh into a bowl. Discard the skins. (Use any leftover potato in another dish.)
Mash the potato until smooth and add the egg yolks, flour, parmesan, salt and the chopped marjoram. Stir together until the mixture comes together as a dough.
Increase the oven to 200C/400F/Gas 6.
Divide the mixture into four, and then on a floured surface, roll each piece into a long sausage shape, about 1¼cm/½in in diameter.
With a sharp knife, cut each sausage into 2½cm/1in pieces of gnocchi (each gnocchi will look like a small pillow).
Bring a large saucepan of salted water to the boil. Drop the gnocchi into the water and cook for 1-2 minutes, or until they float.
Remove from the water with a slotted spoon and place onto a plate.
Meanwhile, heat 25g/1oz of the butter in an ovenproof frying pan until foaming and fry the salmon, skin-side down, for 2-3 minutes. Carefully turn the salmon over and roast in the oven for 5-8 minutes, or until just cooked. Remove the pan from the oven and set aside to rest for five minutes.
In a separate frying pan, heat 25g/1ozif butter until foaming, then add the chestnuts and butternut squash and cook for 3-4 minutes, or until the butternut squash is tender.
Add a little cooking water from the gnocchi to the pan, then whisk in the remaining 25g/1oz butter until the sauce thickens slightly. Season, to taste, with salt and freshly ground black pepper. Add the cooked gnocchi and continue to heat until the gnocchi is warmed through.
To serve, spoon the gnocchi into large serving bowls and top with the salmon.