Put wild rice, salt, and 8 cups cold water in a medium pot. Bring to a boil, lower heat to maintain a steady simmer, and cook until rice is tender, about 45 minutes. Drain and set aside.
Meanwhile, put porcini in a small bowl and pour in 1 1/2 cups boiling water. Let sit until soft, about 15 minutes.
In a large pot, cook 1 tbsp. butter and pancetta over medium-high heat until the meat renders some of its fat and turns a lighter pink. Add button mushrooms and leek. Cook, stirring occasionally, until mushrooms give off their liquid, about 10 minutes.
Meanwhile, lift porcini from liquid with a slotted spoon (reserving liquid), chop finely, and add to pot.
Sprinkle vegetables and pancetta with flour and cook, stirring constantly, until flour starts to stick to the bottom of the pot (scrape it up as much as possible while stirring). Add wine, reserved liquid from soaking porcini (pouring slowly so as to leave any grit behind), and broth. Bring to a boil, then lower heat to maintain a steady simmer and cook 15 minutes.
Meanwhile, combine remaining 4 tbsp. butter, the parsley, and pepper. Set aside.
Add reserved wild rice to vegetable mixture and cook 10 minutes. Stir in cream and cook until hot, about 1 minute. Divide soup among 8 bowls and serve hot, with a dollop of parsley butter on each serving.