Generously dab grated Parmigiano Reggiano onto buttered bread. This will give the panini an extra savory flavor in each bite.
Spread basil pesto on other side of each slice of bread.
Place one slice of bread onto panini press, buttered side down. If you don’t have a panini press, a griddle or skillet will work as well.
Add mozzarella, pepperoni slices, mushrooms, Italian seasoning, chicken, and additional Parmigiano Reggiano and top with other slice of bread, buttered side out.
Close panini press and let sit for 2 minutes. If using a skillet or griddle, you’ll need to adjust your time accordingly.
Remove panini from press and sprinkle Parmigiano Reggiano on top for a little extra flavor.
Dip sandwich in marinara as desired for maximum flavor.