1 cup roasted red pepper strips or 4 thin tomato slices (optional)
Spread butter over 1 side of each piece of bread. Put 4 slices bread onto a sheet of waxed paper, buttered side down.
Top each with 1 slice cheese, 8 slices pepperoni, and, if desired, 1/4 cup of the peppers or tomato slice and remaining 1 slice cheese.
Close sandwiches with remaining bread, buttered side up.
Heat a large nonstick skillet over medium heat. Add 2 sandwiches; cook 4 to 5 minutes per side or until cheese is melted and sandwiches are golden brown. Repeat with remaining 2 sandwiches. (Or, cook sandwiches in 2 batches in a preheated Panini maker or well-oiled waffle iron for 3 to 4 minutes.)