Heat the marinara sauce in a small saucepan over low heat until warmed through.
Lightly brush both sides of the garlic bread or ciabatta with olive oil or melted butter.
Lay two slices of bread on a clean surface.
Divide the grilled chicken slices evenly between the bread slices.
Spoon a generous amount of marinara sauce over the chicken.
Add the mozzarella cheese slices on top.
Add fresh basil leaves if using.
Top with the remaining bread slices.
Place the assembled Chicken Parm Panini in a preheated panini press or grill pan.
Cook for 4–5 minutes, or until the bread is golden brown and crispy and the cheese is fully melted.
Remove from the press and let rest for 1 minute.
Slice in half and serve immediately with extra marinara sauce for dipping.