Preheat oven to 400 degrees. Using a witch's hat cookie cutter, cut out a hat from 4 slices of cheese; set aside.
Spread one side of each slice of bread with butter. Spread the unbuttered side of 4 slices of bread with the mustard. Place on a baking sheet, buttered-side down.
Divide remaining cheese, plus any scraps, between the slices of bread on the baking sheet. Top with remaining bread, buttered-side up.
Place sandwiches in oven and cook 10 minutes, or until cheese has melted. Place a cheese hat on top of each sandwich. Bake until just melted, about 2 minutes. Serve with celery broomsticks.