Combine cheese, butter, mustard and salt in small mixing bowl. Mix on medium speed of mixer until well blended. Gradually stir in flour until well combined. Shape into a log 11 inches long and 2 inches in diameter; roll in chives. Wrap in waxed paper and chill several hours until firm. Preheat oven to 350°. Cut into 1/4-inch thick slices. Bake on unbuttered
baking sheets 12 to 15 minutes. Serve immediately.