How to Make It
Melt 1/2 tablespoon of the butter in a large, deep skillet. When the foam subsides, add the bread crumbs and cook over moderately high heat, stirring, until golden and crisp, 4 to 5 minutes. Transfer the bread crumbs to a plate and wipe out the skillet with a paper towel.
In a large pot of boiling salted water, cook the pasta until just barely al dente. Drain the pasta, reserving 1 1/2 cups of the pasta cooking water.
Meanwhile, melt the remaining 3 1/2 tablespoons of butter in the skillet. Add the mushrooms, season with salt and pepper and cook over high heat, stirring, until tender and lightly browned, about 5 minutes. Add the cream and cook over moderate heat, scraping up any browned bits from the bottom of the pan, until almost evaporated, about 2 minutes. Stir in the mascarpone and 3/4 cup of the reserved pasta cooking water and cook until a creamy sauce forms.
Add the pasta to the sauce and toss gently over moderate heat until fully coated and cooked through, about 3 minutes; add the remaining 3/4 cup of pasta cooking water as needed to keep the pasta moist. Transfer to a large, warmed bowl, sprinkle with the toasted bread crumbs and chives and serve.