Heat broiler, with rack set 4 inches from heat. In a medium saucepan, cook chicken and barbecue sauce over medium until heated through, 6 to 8 minutes.
Meanwhile, in a small bowl, combine butter and garlic; season with salt and pepper. Spread one side of each slice of bread with garlic butter. Place on a baking sheet, buttered side up. Broil until golden brown, 1 to 3 minutes, then turn over, and broil until light brown, about 1 minute longer. To serve, sandwich chicken mixture between slices of bread.