Heat oven to 425'F. Grease large baking sheet. In large bowl, combine flour, sugar, baking powder, salt, and baking soda. With pastry blender or 2 knives, cut the butter in until mixture resembles coarse crumbs. Remove 1 1/4 C flour mixture to medium-size bowl.
In small bowl, combine buttermilk and vanilla. In cup, combine 1/4 C buttermilk mixture and the chocolate. Add remaining buttermilk mixture to remaining flour mixture in large bowl and mix lightly with fork until it clings together and forms a soft dough. Add chocolate mixture to the reserved 1 1/4 C flour mixture in medium-size bowl and stir with fork until it forms a soft dough.
Turn both doughs out onto lightly floured surface; divide chocolate dough into 6 pieces. Dot 3 chocolate pieces over surface of plain dough. Fold plain dough over and dot with remaining 3 pieces chocolate dough; knead 5 or 6 times to marbleize dough. Divide dough in half. With lightly floured rolling pin, roll one half of dough into a 7-inch round. Cut into 4 wedges. Repeat with remaining half of dough.
Place scones, 1 inch apart, on greased baking sheet. Pierce tops with tines of fork.
Bake scones 15 to 18 minutes or until golden brown. Serve warm.