1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
2. Bring a large pot of water to boil. Once water is boiling add two tablespoons salt and pasta.
3. Meanwhile, butter a nine- by 13-inch dish or aluminum pan. Directly to the pan add tomato sauce, ricotta cheese, two tablespoons of the butter, two cups shredded cheese, salt, pepper and red pepper flakes.
4. Before draining pasta, remove 1/2 cup pasta water and add to the pan with the rest of the ingredients. Stir to combine.
5. Add cooked pasta to the pan and stir to fully coat every single piece.
6. Top the pasta with one to two cups shredded mozzarella (depending on how cheesy you like it).
7. Scatter remaining tablespoon of cubed butter over the top.
8. Bake for 40–45 minutes until the top is golden brown and bubbly.