Preheat the oven to 160C/140C Fan/Gas 3. Bake the potatoes on a tray for 1 hour until cooked through. (Test by piercing a small knife or fork into the centre of the potato. When soft they are done.)
Meanwhile, to make the mushroom duxelle filling, heat the butter in a pan. Gently fry the mushrooms, shallots, garlic, thyme and allspice. Season with salt and pepper to taste. Cook until the liquid that comes out of the mushrooms has evaporated and the mixture is almost dry. Set aside and leave to cool.
Once cool enough to handle, roll into 2cm/¾in balls. Chill in the fridge until the potatoes are cooked.
Once the potatoes are cooked, peel them and pass through a potato ricer or use a potato masher to create a smooth mash.
Add the flour, egg, egg yolks and salt, and mix. Divide the dough into walnut-sized portions. Place the prepared mushroom filling into the centre of each dough ball. Cover your hands in flour, and carefully work the dough around the mushroom mixture until it is completely encased within the dough. Each dumpling should be about the size of a golf ball.
Cook the dumplings in a large pan of boiling water. Simmer until they start to float to the surface. This can be a little tricky if some of them have air pockets, but they should be ready in around 6–8 minutes. Once cooked, strain the dumplings and leave to cool slightly.
Heat a knob of butter in a large pan and carefully sear the dumplings. Remove from the pan and set aside.
Mix the defrosted lingonberries with the sugar and set aside.
Heat a little more butter in a pan and gently fry the mushrooms for a couple of minutes. Remove from the pan and set aside.
In the same pan, melt 50g/1¾oz of butter over a high heat until it starts to smell nutty and turn brown. Turn off the heat and remove from the pan so it doesn’t burn.
Divide the dumplings between 4 plates, garnish generously with the wild mushrooms and lingonberries, and pour the brown butter over to finish.