Preheat oven to 375°. In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolk. Gradually add flour until blended. , Shape into 1-in. balls. In a small bowl, beat egg white. Dip balls in egg white, then roll in pecans. Place 2 in. apart on ungreased baking sheets; flatten slightly. Bake at 375° for 8-12 minutes or until edges are lightly browned. Remove from pans to cool on wire racks.