Preheat oven to 180ºC . Line cookie sheets whit foil. In large mixing bowl, combine butter, brown sugar, egg, flour and vanilla. Beat until well mixed. Stir in toasted sesame seeds. Drop dough on foil line baking sheets, using half teaspoon for each cookie. Space 5 cm apart. Bake 8-10 minutes or until cookies are lightly browned. Remove from sheets immediately and cool on racks.