Position a rack in the bottom of the oven, set a sheet pan on top, and preheat the oven to 450°F.

Spread 6 slices of bread with half of the butter and place, buttered sides down, on a second sheet pan.

Spread each bread slice on the sheet pan with 1 to 2 tablespoons marinara sauce and top with 2 slices of cheese. Top with pepperoni and the remaining bread slices. Spread the tops with the remaining butter.

Carefully remove the hot sheet pan from the oven and place it on top of the sandwiches, rimmed edges up.

Place the stacked baking sheets in the oven on the bottom rack. Bake until the cheese is melted and the bread is golden brown and crispy, 10-12 minutes. Meanwhile, warm the remaining marinara in a microwave or on the stove.

Remove sandwiches from the oven and serve immediately with remaining warmed marinara sauce. Refrigerate any leftover sandwiches up to 2 days and reheat in a regular oven or toaster oven at 325°F for 5-10 minutes.