1 lb. boneless, skinless, chicken breast halves, seasoned with salt and pepper
1 1/2 c. heavy cream
2 cloves garlic, chopped
1 stick unsalted butter (8 Tbsp.)
1 1/2 c. grated Parmesan, divided
2 tsp. black pepper
4 Tbsp. chopped fresh parsley, divided
Preheat broiler with rack 5 inches from element. Spray two 24-cup mini muffin tins with nonstick spray; set aside. Bring a large post of salted water to a boil.
Cook shells in boiling water until al dente, 12-15 minutes; drain and rinse under cold water.
Heat oil in a skillet over medium heat. Add chicken and saute until cooked through, 8-9 minutes per side; cool slightly, then chop into small pieces.
Warm cream and garlic in a large saucepan over medium-low heat until steaming. Whisk in butter until melted. Add 1 c. Parmesan, 2 tsp. pepper, and 2 Tbsp. parsley. Increase heat to medium-high and simmer until reduced to 1 cup, about 10 minutes. Stir in chicken and remove from heat.
Fill each shell with 1 heaping Tbsp. chicken mixture. Stagger filled shells, open ends up, in every other muffin tin well to prevent them from sticking to each other. Sprinkle shells with remaining 1/2 c. Parmesan.
Broil shells until sauce bubbles and Parmesan melts and is golden, about 2 minutes. Garnish shells with remaining 2 Tbsp. parsley.