Heat oven to 350°F. Grease 2 large baking sheets. In medium bowl, combine flour, cocoa, and baking soda. In large bowl, with mixer on high speed, beat together butter and sugar until light and fluffy, about 3 minutes. Add egg and vanilla, and beat to combine. Alternate adding flour mixture and buttermilk in small batches, beginning and ending with flour mixture. Mix until smooth, occasionally scraping sides of bowl.