To make the bacon filling, heat the butter in a pan. Fry the shallots and thyme leaves until soft, then add the allspice. Now add the bacon and fry until cooked through and golden. Set to one side to cool.
Divide the dumpling dough into 12 (approx 45g each) on a floured work surface, press the dough into a disc shape. In the centre of each disc place a teaspoon of filling. Gather up the dough and press together to make a ball. Slightly flatten and set to one side. Continue with the remainder.
Boil the dumplings slowly in a pot of lightly salted water without a lid for 5–6 minutes, the dumplings will rise to the surface when they’re done. Place on a plate.
Mix the defrosted lingonberries with the sugar and set aside.
Heat a little more butter in a pan and gently fry the mushrooms if using for a couple of minutes. Remove from the pan and set to one side to keep warm.
Steam the cavolo nero in a little water in a saucepan for several minutes. Once done add a knob of butter and black pepper.
In a heavy based pan, heat a knob of butter and carefully sear the dumplings. Remove from the pan and set aside to keep warm. Melt 50g of butter in the same pan 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 with the wild mushrooms if using, cavolo nero and lingonberries, and pour the brown butter over and sprinkle with hazelnuts to finish. Serve.