Combine mayonnaise, grated Parmesan cheese, lemon juice, pesto, lemon zest and pepper in a small bowl.
Mix together and spread over 2 bread slices.
Layer with chicken, bacon, mozzarella cheese and tomato.
Add remaining mayo mixture to the other 2 pieces of bread and place on top of each sandwich.
Brush melted butter onto the outsides of each sandwich and cook on a panini maker or indoor griddle for about 3-4 minutes (or until sandwiches are browned and cheese is melted).
Serve warm.