Sift together all-purpose flour, baking soda, and salt in a large mixing bowl. Cut in butter with a pastry blender until mixture resembles coarse meal. Stir in whole wheat flour, mixing well. Add buttermilk, stirring until the dry ingredients are thoroughly moistened.
Turn dough out onto a lightly floured surface, and knead about 5 minutes. Shape dough into a round loaf; place on a greased baking sheet. Using a sharp knife, cut a cross 1/4-inch deep on top of loaf; lightly sprinkle cross with flour.
Bake at 400° for 40 minutes or until bread sounds hollow when tapped. Remove from baking sheet, and cool completely on wire rack.