2 bunches fresh chives, very thinly sliced (3/4 cup)
Special equipment: a 2-inch round cookie cutter
Directions
Stir together Cheddar, mayonnaise, onion, and salt and pepper to taste.
Cut out a round from each bread slice with cookie cutter. Arrange 8 bread rounds on a work surface and divide half of Cheddar mayonnaise among them, spreading evenly to edges. Top each with a plain round. Cover sandwiches with plastic wrap and make more in same manner.
Spread a thin layer of butter around cut edges of rounds and roll in chives, pressing with your fingertips.