With a 4 1/2-in. or 3 3/4-in. pumpkin-shaped cookie cutter (or both), cut pumpkin shapes from bread slices. (Save crusts for another use, such as bread pudding.)
Layer half of slices with cheese, using about 2 tbsp. cheese for large slices and 1 1/2 tbsp. for smaller ones. Sprinkle with chile flakes or parsley if you like.
Top cheese with remaining halves of bread and spread top with mayonnaise, using 1 to 1 1/2 tsp. per sandwich. Evenly sprinkle each sandwich with 1 1/2 to 2 tbsp. cheese and press down to help everything stick together.
Put 1 tbsp. butter in a large nonstick frying pan and melt over medium heat. When the foam has just subsided, add as many sandwiches as will fit without crowding, mayo-cheese side down. Using the back of a soup spoon, spread tops with more mayonnaise and sprinkle with more cheese. Press down to help everything stick together.
Cook sandwiches over medium heat until golden brown underneath and cheese is starting to melt in middle, at least 5 minutes. Carefully turn each sandwich over and cook until second side is browned, 2 to 3 minutes more. Scoop onto a cutting board and trim edges if they've gotten wild. Repeat with rest of butter and sandwiches, turning heat down if necessary.
Serve hot, with tomato soup if you like.