Step 1Bring a large pot of salted water to a boil over high heat. Add the fusilli and cook until al dente, about 12 minutes.
Step 2While the pasta cooks, melt 1 tablespoon of the butter and the olive oil over medium heat in a large skillet. Add the onion and cook until the onion is softened, about 3 minutes. Add the garlic and cook, stirring, until the garlic is very fragrant, about 1 minute. Lower the heat to medium-low and add the tomatoes, whipping cream, mascarpone and vodka. Cook, stirring frequently until the mixture has thickened slightly, about 10 minutes. Stir in the basil, red pepper flakes and salt and pepper to taste.
Step 3Drain the pasta, reserving about 1 cup of the cooking water.
Step 4Add the pasta to the skillet. Mix well and add enough pasta cooking water and the 1/4 cup of grated Parmesan to make a rich sauce. Stir in the remaining 2 tablespoons of butter. The sauce should coat the pasta and look silky, but not be soupy. Taste and season with salt and pepper if needed. Pass additional grated cheese.