Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, about 10-12 minutes. Drain and set aside.
While the pasta cooks, heat 2 tablespoons of butter in a large skillet over medium heat.
Add the minced garlic and crushed red pepper flakes to the skillet. Sauté for about 1 minute until fragrant, stirring constantly to prevent burning.
Add the shrimp to the skillet in a single layer. Season with salt and black pepper. Cook for 2-3 minutes on each side until the shrimp turn pink and are cooked through. Remove shrimp from the skillet and set aside.
In the same skillet, add the remaining 2 tablespoons of butter and let it melt over medium heat.
Return the cooked shrimp to the skillet and add the cooked penne pasta. Toss everything together to coat evenly in the garlic butter sauce.
Stir in the fresh lemon juice and grated Parmesan cheese. Toss again until the cheese is melted and well distributed.
Remove from heat and sprinkle chopped fresh parsley over the top.
Serve immediately while hot.