Step 1: Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve about 1 cup of pasta water before draining.
Step 2: While the pasta is cooking, melt 4 tablespoons of unsalted butter in a large skillet or Dutch oven over medium heat. Add the olive oil.
Step 3: Add the minced garlic to the skillet and sauté for 1-2 minutes until fragrant. Be careful not to burn the garlic, as it can turn bitter.
Step 4: Stir in the red pepper flakes (if using) and continue to cook for another 30 seconds.
Step 5: Deglaze the pan with the white wine (if using), scraping up any browned bits from the bottom. Let it simmer for 1-2 minutes to cook off the alcohol.
Step 6: Add the cream cheese or mascarpone (if using) and stir until completely melted and smooth.
Step 7: Gradually add about 1/2 cup of the reserved pasta water to the sauce, stirring constantly until it reaches your desired consistency. You might need a little more or less.
Step 8: Drain the cooked pasta and add it directly to the skillet with the sauce. Toss well to coat every strand.
Step 9: Stir in the remaining 2 tablespoons of butter, grated Parmesan cheese, and fresh parsley. Continue to toss until the cheese is melted and the pasta is evenly coated.
Step 10: Taste and adjust seasonings. Add salt and black pepper as needed. If the sauce seems too thick, add another splash of pasta water. Serve immediately with extra Parmesan and fresh parsley for garnish.