Place bread on work surface and spread butter on each slice. Turn bread over and spread marinara sauce on each slice. Place 2 slices each of the mozzarella cheese on 4 slices of the bread and 3 slices each of pepperoni. Top with remaining 4 slices of bread.
Place sandwiches on griddle, flip when bread is toasty golden and continue to cook until the other side is golden and cheese is melty. Cut in half & serve.