Mix brown sugar, margarine, peanut butter, egg and vanilla. Stir in flour, baking soda and salt. Cover and refrigerate at least 2 hours.
Heat oven to 350 degrees F. Shape dough into 1-inch balls; roll in cereal. Place about 2 inches apart on ungreased cookie sheet. Bake until almost no indentation remains when touched, 12 to 13 minutes.
