Preheat oven to 350°F. Bring large pot of salted water to boil and cook rotini according to package directions. Reserve 1/2 to 3/4 cup of the pasta water before draining. Drain pasta and lightly rinse with cool water to stop cooking process. Set aside.
Meanwhile, heat large saucepan over medium heat. Season chicken breasts with salt and pepper on both sides. Add 1 tablespoon butter and 1 tablespoon olive oil to pan. Heat until bubbling heavily; add half the chicken slices in single layer and sauté until deep golden brown on one side. Flip; cook other side until golden brown, about 2 minutes each side. Repeat until slices have been cooked. Set aside.
Melt 4 tablespoons butter in large pot over medium heat. Sprinkle in flour; whisk to form a paste. When bubbly and golden brown, slowly whisk in milk, stirring constantly until mixture simmers. Remove from heat. Add 2 heaping cups shredded mozzarella; stir until melted. Mix in pasta. If mixture is too thick, add some of reserved pasta water. Stir in chicken.
Pour half the mac and cheese into buttered single serving dishes or 9x13-inch baking dish. Distribute half the marinara sauce evenly over the top. Layer with remaining macaroni and cheese and marinara sauce.
In small bowl, melt remaining butter. Stir in bread crumbs and parmesan. Sprinkle crumbs over macaroni. Bake 10-15 minutes for single servings or 15-20 minutes for larger portions, until crumbs are golden brown and the macaroni is heated through. Garnish with basil.