Heat oven to 350°F. In large bowl, beat brown sugar and butter with electric mixer on medium speed until light and fluffy, scraping bowl occasionally. Beat in vanilla and egg yolk until blended. On low speed, beat in flour until mixture forms a smooth dough.
Drop dough by rounded teaspoonfuls 2 inches apart onto ungreased cookie sheets.
Bake 10 to 15 minutes or until light golden brown and set. Immediately remove from cookie sheets.