Heat the marinara sauce in a small saucepan over low heat until warmed through.
Lightly brush both sides of the garlic bread slices or ciabatta rolls with olive oil or melted butter.
Lay two slices of bread on a work surface. Add a layer of sliced grilled chicken.
Spoon marinara sauce evenly over the chicken.
Place mozzarella slices on top of the sauce.
Add fresh basil leaves if using.
Top with the remaining bread slices to complete the sandwich.
Place the assembled Chicken Parm Panini in a preheated panini press or grill pan.
Cook for 4–5 minutes, pressing gently until the bread becomes golden brown and crispy and the cheese melts completely.
Remove from heat, slice in half, and serve immediately with extra marinara sauce for dipping.