Mashed Potato Stuffed Shells

Name: Mashed Potato Stuffed Shells
Ingredients: 1 (12-ounce) box jumbo pasta shells; 1 1/2 pounds medium red potatoes; 4 tablespoons (1/2 stick) unsalted butter; 1 tablespoon plus 1 1/2 teaspoons dried parsley; 1 tablespoon garlic powder; 3/4 cup whole milk, plus more as needed; 3/4 ounce Parmesan cheese, finely grated (1/4 cup); 3/4 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 2 tablespoons olive oil, plus more for the pasta and baking dishes; Marinara sauce, for serving (optional)