Cook the pasta. In a large pot of salted boiling water, cook the penne pasta until al dente or one minute less than al dente, as it will continue cooking in the sauce. Drain and set aside.
Cook meat and add sauce. In an oven-proof skillet over medium heat, cook the ground beef until it is no longer pink, breaking it up as it cooks. If there is excess grease, drain some of it off (if using 85-90% lean beef, this step may not be necessary). Next, add the jarred marinara sauce to the cooked beef in the skillet. Stir well and let it simmer for 3-5 minutes to blend the flavors.
Add cream and Parmesan. Stir in the heavy cream and Parmesan cheese. Mix until the sauce is heated through and begins to simmer again.
Preheat and add pasta. Adjust the oven rack so that it is 2 inches away from the broiler and preheat it to high. Add the cooked pasta to the skillet with the sauce. Stir to combine and simmer for 1 minute to allow the pasta to absorb some of the sauce.
Layer cheese on top. Sprinkle the shredded mozzarella evenly over the top of the pasta mixture. Cut the salted butter into small pats and distribute them evenly over the cheese.
Broil until golden. Place the skillet under the broiler. Broil for 1-2 minutes, keeping a close eye on it to prevent burning, until the cheese is melted and golden brown.