In another small bowl, mix flour and baking soda; set aside. In medium bowl, beat 1/4 cup butter and the shortening with electric mixer on medium speed until fluffy. Gradually add 3/4 cup of the granulated sugar, beating well. Add egg yolks, one at a time, beating until blended after each addition. Stir in 2 teaspoons vanilla bean paste. Add flour mixture alternately with buttermilk, beating on low speed after each addition until blended.