1. Arrange one cheese slice on each of four slices of bread; spread evenly with pizza sauce. Top each with another cheese slice, and spread evenly with pesto sauce. Arrange 20 pepperoni slices over pesto sauce; top with remaining cheese slices and remaining bread slices.
2. Spread half of butter on tops of sandwiches. Invert sandwiches onto a hot nonstick skillet or griddle, and cook over medium heat until browned. Spread remaining butter on ungrilled sides of sandwiches; turn and cook until browned. Serve immediately.