Put a pot of salted water on to cook pasta. While pasta is cooking (according to package directions, make the sauce:
Heat butter and oil in a 12″ skillet over medium-high heat. Add sliced shallot. Season to taste and cook until soft, about 2 minutes, stirring frequently.
Add poppy seeds and cook, stirring, until fragrant and shallots begin to brown lightly, about 3 minutes.
Add wine and cook until almost all liquid evaporates, about 5 minutes.
Add buttermilk and cream; cook, stirring, until reduced slightly, about 3 minutes. When pasta has cooked to almost al dente, drain it, reserving a little cooking water, and add the cooked pasta to sauce in skillet.
Add prosciutto, cheese, and lemon juice; toss to combine, adding water if necessary to make a smooth sauce. Adjust seasoning and transfer to two plates. Sprinkle with scallions and more Parmesan.