Combine the ground beef, Worcestershire, mustard, onion, salt and pepper in a large bowl. Set aside. Combine the cheese and butter, and blend. Divide the hamburger mixture into 12 patties. Place a spoonful of the cheese-butter mixture on 6 of the patties. Place the remaining six patties on top of the cheese-butter mixture. With your fingertips, pinch the edges together to seal in the filling.
Place the filled hamburgers on an oiled grill over medium heat. Cook for about 4 minutes on each side for rare, or 6 to 7 minutes for well done. Serve with a sprinkling of snipped chives.
Makes 6 servings