Makes 6 sandwiches
Preheat oven to 250 degrees.
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 saute 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 for another 4 to 5 minutes. 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.