Combine the porcini and broth in a small saucepan and heat to boiling. Cover and simmer over low heat for 10 minutes. Remove from heat and let stand, covered for 20 minutes.
Drain the porcini through a sieve lined with a dampened paper towel and set aside the broth separately. Rinse the mushrooms under running tap water and rub with your fingers, cut off and discard any gritty pieces. Drain well and pat dry. Finely chop and set aside.
Return the strained broth to the rinsed-out saucepan and heat to boiling. Adjust the heat to maintain a steady simmer.
Heat 2 tablespoons of the butter and the olive oil in a heavy saucepan over low heat until the butter is melted. Add the shallots and sauté, stirring, until tender, about 3 minutes. Add the rice and sauté over low heat, stirring for 2 minutes.
Add enough of the hot broth (about 1/2 cup) to just cover the rice. Adjust the heat to maintain a steady simmer and cook the rice, stirring constantly, until almost all the broth has been absorbed, about 3 to 4 minutes.
Add another ladle of broth (about 1/2 cup). Cook stirring constantly, until all the broth has been absorbed. Continue adding the broth, about 1/2 cup at a time, and cooking and stirring. Add the next ladleful when the broth is absorbed and the rice begins to pull away from the sides of the pan as it is stirred.
About halfway through the cooking time add the reserved chopped porcini. Cook until the rice is creamy and firm but not chalky in the center when it is tasted. The total cooking time is 25 to 35 minutes or more.
Remove from the heat. Stir in the Parmesan and salt. Sprinkle generously with black pepper. Serve at once with additional cheese on the side.