Preheat oven to 250º F.

Arrange bread slices on a work surface and brush one side of each slice with 1/2 tablespoon butter. Turn the buttered sides face down. Top 6 slices bread with American cheese and 1/2 cup cheddar cheese and close sandwiches with the remaining slices of bread so buttered sides both face out.

In a large sauté pan, cook half the sandwiches over medium-high heat, turning once bread is lightly toasted and cheese is melted, 4 to 5 minutes. Continue to cook sandwiches until golden brown on the second side, 3 to 4 minutes longer. Transfer sandwiches to a cutting board and, using a witch's hat or other Halloween-themed cookie cutter, cut out sandwiches. Discard crusts. Transfer to a baking sheet or an ovenproof platter and place in a warm oven while the remaining sandwiches are cooking. Serve immediately.