1. Sift flour, soda and salt onto wax paper.
2. Beat butter or margarine with sugar until fluffy in a large bowl with electric mixer at high speed; beat in egg and vanilla.
3. Stir in flour mixture, half at a time, blending well to make a soft dough. Chill several hours, or overnight, until firm enough to handle.
4. Roll dough, a teaspoonful at a time, into balls between palms of hands, then roll in sesame seeds in a pie plate to coat lightly. Place, 2 inches apart, on lightly greased large cookie sheets.
5. Bake in moderate oven (350°) 10 minutes, or until delicately golden Remove from cookie sheets to wire racks with spatula; cool completely.