Bring a large pot of salted water to a boil. Add penne pasta and cook according to package instructions until al dente, about 10-11 minutes. Drain and set aside.
While the pasta cooks, heat 2 tablespoons of butter in a large skillet over medium heat until melted.
Add the minced garlic and crushed red pepper flakes to the skillet. Sauté for about 1 minute until fragrant, stirring constantly to avoid burning the garlic.
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 to combine and coat the pasta and shrimp evenly with the garlic butter sauce.
Stir in the grated Parmesan cheese, fresh parsley, and lemon juice. Toss again until everything is well mixed and heated through, about 1-2 minutes.
Remove from heat and serve immediately.