Preheat oven to 350°F. Cook pasta in a large pot according to its directions. Drain and set aside.
In a large oven-safe skillet or Dutch oven melt 1 Tbsp butter with garlic. Once melted and garlic begins to create an aroma, add shrimp and fresh parsley. Season with a dash of salt.
Cook shrimp until it is no longer opaque. Shrimp should be pink. Remove shrimp and set aside on a plate, keeping the butter and juices in the skillet/Dutch oven.
Add the rest of the butter, letting it melt completely. Make a roux by stirring in the flour and cook for about 1 minute. Stir in the milk and broth and bring to a simmer. Stir in ¾ cup of the mozzarella and the Parmesan, seasoning with salt and pepper.
Stir until sauce is smooth. If you prefer a thinner sauce, add more milk, if you prefer thicker, add more cheese or a little flour. Add shrimp and mostaccioli until evenly combined.
Sprinkle the remaining mozzarella on top and bake until cheese is melted, about 5 or so minutes.
Serve with garlic bread or breadsticks...delicious!
