Preheat the oven to 450F; set rack in middle level. Prepare biscuit dough. COMBINE THE FLOURS, salt and baking powder in a mixing bowl and stir well to mix. Rub in the butter by hand or with a pastry blender until the mixture is mealy. Stir in 3/4 cup of the buttermilk with a fork and continue stirring gently until the dough begins to hold together. (If the dough is dry, add more buttermilk, 1 tablespoon at a time.) Sprinkle the work surface generously with flour and scrape the dough onto it. Fold the dough over on itself two or three times. Use the dough immediately . Pat dough into a 6-by-12- inch rectangle and cut into eight 3-inch biscuits with a sharp, floured knife. Transfer to a parchment-lined cookie sheet. Paint tops with buttermilk. Bake for 10 to 15 minutes, until well risen and golden.