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