In medium bowl with electric mixer at medium speed, beat brown sugar, peanut butter, margarine and egg until fluffy; blend in cereal and vanilla. Stir in flour and baking soda to make a stiff dough.
Shape dough into 1" balls. Place 2" apart on ungreased baking sheets. Flatten balls with bottom of floured glass; press with fork tines to make criss-cross pattern. Bake at 350'F. for 8-9 minutes or until lightly browned. Remove from sheets; cool on wire racks.