Place 1 cheese slice on half of each tortilla; top each with 8 pepperoni slices and another slice cheese. Fold tortilla in half.
Brush both sides of each folded tortilla with butter.
Heat 12-inch skillet or griddle over medium heat. Place 2 quesadillas into skillet; cook, turning once, 4-6 minutes or until golden brown and cheese is melted. Repeat with remaining quesadillas.
For flavor variety, use cooked and crumbled Italian sausage instead of pepperoni or add chopped red or green bell peppers.