Cover porcini with boiling water. Let sit 30 minutes.
Meanwhile, in a large skillet over moderate heat, melt butter in olive oil.
Add white mushrooms, 1/2 teaspoon salt and 1/4 teaspoon pepper.
Cook until almost dry and mushrooms are tender, about 30 minutes.
Cook and drain pasta according to package directions.
While pasta is cooking, finish sauce:
Remove porcini mushrooms from soaking liquid; rinse, chop and add to skillet.
Place a coffee filter in a small strainer and pour mushroom soaking liquid through it.
Add strained liquid to skillet.
Cook mushroom mixture over high heat until mostly evaporated but still saucy,
about 5 minutes.
Stir in sour cream, remaining 1/2 teaspoon salt, 1/4 teaspoon pepper,
paprika and parsley.
Cook until heated through, about 2 minutes.
Toss with pasta and mushrooms, serve immediately.