Combine cheese, butter, mustard and salt in small mixing bowl. Mix on medium speed with electric mixer until well blended. Gradually stir in flour until well combined. Shape into 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.