Spray a waffle iron with cooking spray and preheat according to manufacturer's instructions.

Arrange 4 bread slices on a work surface.

Spread 1 tablespoon of pesto onto each bread slice.

Drizzle 1 teaspoon of balsamic vinegar onto each of the remaining 4 bread slices.

Arrange 1 mozzarella cheese slice and 1 ham slice on each pesto-covered bread slice.

Top with a balsamic vinegar-drizzled bread slice to form a sandwich.

Cook each sandwich in the preheated waffle iron until the cheese is melted and the bread is golden brown, approximately 3 to 5 minutes.

Spray the waffle iron with additional cooking spray before cooking each sandwich.