Spread 1/2 tablespoon of butter on one side of each of the 8 slices of bread. Spread 1 tablespoon of marinara of the other side of each bread.
Top each slice of bread on the marinara side with the shredded mozzarella cheese.
Divide up the mini pepperoni slices and place them on only 4 slices of the bread, on top of the marinara and mozzarella cheese. I suggest slightly pressing down on the toppings on each slice of bread to make sure are stuck in there and don't fall out too easily.
Take the slice of bread with marinara and cheese only and place it on top of the slice of bread with marinara, cheese and pepperoni. The buttered sides should be facing out, with the marinara, sauce, cheese and mini pepperoni all in the middle.
Preheat a medium sized pan over medium high heat. When pan is hot, add assembled sandwiches to the pan and cook for 2-3 minutes each side until outsides are browned and cheese is melted.