Melt 1/2 tablespoon of butter in a large skillet over medium-high heat. Add the breadcrumbs and cook, stirring frequently, until golden and crisp (about 4-5 minutes). Transfer to a plate and wipe the skillet clean.

Cook the pasta in a large pot of boiling salted water until just barely al dente. Reserve 1 1/2 cups of the pasta cooking water before draining.

In the same skillet, melt the remaining 3 1/2 tablespoons of butter over high heat. Add the mushrooms, season with salt and pepper, and cook, stirring occasionally, until tender and golden brown (about 5 minutes).

Pour in the heavy cream and scrape up any browned bits from the bottom of the pan. Let it cook for 2 minutes, until slightly thickened.

Stir in the mascarpone and 3/4 cup of the reserved pasta water. Cook over low heat until combined into a velvety sauce.

Toss the pasta into the skillet with the sauce, stirring gently to coat evenly. Add more pasta water (up to 3/4 cup) to keep the consistency smooth and creamy. Cook for another 3 minutes.

Transfer the pasta to a large serving bowl, top with breadcrumbs and snipped chives, and serve immediately.