Cook the large pasta shells in boiling salted water until al dente, about 10 minutes. Drain and set aside to cool slightly.
In a skillet, melt 2 tablespoons of butter with a splash of olive oil over medium heat. Add the minced garlic and sauté until fragrant, about 1 minute, until the garlic is soft and aromatic.
Stir in the cooked chicken and cook for another 2-3 minutes to warm through, then remove from heat and let it cool slightly.
In a mixing bowl, whisk together the heavy cream, grated Parmesan, and remaining 2 tablespoons of butter until smooth. Season with salt and pepper to taste.
Add the garlic chicken mixture into the Alfredo sauce, stirring to combine and create a flavorful filling.
Gently stuff each cooked shell with the garlic chicken Alfredo mixture, then place them seam-side up in a greased baking dish.
Pour the remaining Alfredo sauce over the filled shells, making sure they are well coated. Sprinkle shredded mozzarella evenly on top.
Preheat your oven to 375°F (190°C). Bake the assembled dish uncovered for 20-25 minutes until bubbling and the cheese is melted and slightly golden.
Remove from the oven and let it rest for 5 minutes before serving, allowing the flavors to settle and the dish to firm up slightly.
Garnish with extra Parmesan or chopped fresh herbs if desired, then serve hot and enjoy the creamy, cheesy goodness.
