1 cup (5 ounces) unbleached all-purpose flour
1 cup (4 ounces) plain cake flour
2 teaspoons baking powder
1/2 teaspoon baking soda
1 teaspoon sugar
1/2 teaspoon salt
8 tablespoons (1 stick) cold unsalted butter, cut into 1/3-inch cubes
3/4 cup cold buttermilk, or 3/4 cup plus 2 tablespoons plain yogurt
1. Adjust an oven rack to the middle position and heat the oven to 450 degrees.
2. Place the flours, baking powder, baking soda, sugar, and salt in a large bowl or the work bowl of a food processor fitted with the metal blade. Whisk together or process with six 1-second pulses.
3. If making by hand, use two knives, a pastry blender, or your fingertips to quickly cut in the butter until the mixture resembles coarse meal with a few slightly larger butter lumps. If using a food processor, remove the cover and distribute the butter evenly over the dry ingredients. Cover and process with twelve 1-second pulses.
4. If making by hand, stir in the buttermilk with a rubber spatula or fork until the mixture forms a soft, slightly sticky ball. If using a food processor, remove the cover and pour the buttermilk evenly over the dough. Process until the dough gathers into moist clumps, about eight 1-second pulses.
5. Transfer the dough to a lightly floured surface and quickly form into a rough ball. Be careful not to over mix. Using a sharp knife or dough cutter, divide the dough into quarters and then cut each quarter into thirds. Quickly and gently shape each piece into a rough ball and place on an ungreased baking sheet. (The baking sheet can be wrapped in plastic and refrigerated for up to 2 hours.)
6. Bake until the biscuit tops are light brown, 10 to 12 minutes. Serve immediately.