01 - 
                Preheat your oven to 375°F. Move the rack to the top third of the oven for best browning results.
              
              
              
                02 - 
                Bring a large pot of water to a boil and add salt. Cook the penne for 2 minutes less than the package directions suggest (it will continue cooking in the oven). Drain when done.
              
              
              
                03 - 
                Return the drained pasta to the pot. Add the marinara sauce and about 1/3 of the pepperoni slices. Stir everything together until the pasta is evenly coated.
              
              
              
                04 - 
                Transfer the pasta mixture to a 9x13 inch baking dish, spreading it out in an even layer. Sprinkle the shredded mozzarella evenly over the top, then arrange the remaining pepperoni slices over the cheese.
              
              
              
                05 - 
                Bake uncovered for 20 minutes until the cheese is melted and bubbly. Then switch to broil for 2-3 minutes to crisp up the pepperoni and cheese. Watch carefully to prevent burning!