Step 1: Cook the pasta in a large pot of boiling salted water according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
Step 2: While the pasta is cooking, heat a large skillet over medium-high heat. Add the sausage meat and cook, breaking it up with a spoon, until browned and cooked through (about 7-8 minutes).
Step 3: Reduce heat to medium. Add the butter to the skillet with the sausage. Once melted, stir in the minced garlic and red pepper flakes (if using). Cook for 1-2 minutes until the garlic is fragrant and light golden.
Step 4: Add the drained pasta and half of the reserved pasta water to the skillet. Toss everything together to coat the pasta in the garlic butter sauce.
Step 5: Stir in the Parmesan cheese and chopped parsley. If the pasta seems too dry, add more of the reserved pasta water until a glossy sauce forms. Season with salt and pepper to taste and serve immediately.