Using a stand mixer, mix flour, sugar, baking soda, and salt on low speed just until combined. With mixer running, add softened butter 1 piece at a time. Continue beating until mixture is moist crumbs and well blended, about 1 to 2 minutes. Add buttermilk, egg, yolk and vanilla. Beat on medium speed until light and fluffy, about 1 minute. Scrape down bowl as necessary.