Cook pasta according to package instructions. Drain and set aside.
In a large skillet over medium heat, melt butter. Add garlic and sauté until fragrant (about 1 minute).
Pour in heavy cream. Season with salt and pepper. Stir continuously for 5–7 minutes until sauce begins to thicken.
Stir in grated Parmesan cheese until melted and smooth.
Add red pepper flakes (if using) and the sliced smoked sausage. Heat for 2–3 minutes until sausage is warmed through.
Add the cooked pasta to the skillet. Toss everything until pasta is evenly coated with sauce.
Serve hot, garnished with additional Parmesan and red pepper flakes if desired.