Position racks in the upper and middle thirds of the oven and preheat the oven to 450F.

In a large bowl, stir together 3 1/2 cups of the flour, the masa, baking powder, and salt. With a pastry cutter, blend in the butter and shortening until the mixture resembles a coarse and slightly lumpy meal. Stir in the buttermilk until a soft, crumbly dough is formed. Sprinkle the work surface with half of the remaining flour. Turn the dough out, gather it into a ball, and briefly knead it, just until it holds together. Flatten the dough, sprinkle it with the rest of the flour, and roll it out about 1-inch thick. With a round 3-inch cutter, form the biscuits, transferrring them to 2 ungreased baking sheets and spacing them about 2 inches apart. Gather the scraps into a ball, roll it out to 1-inch thick, and cut out the remaining biscuits.

Set the baking sheets on the racks and bake about 15 minutes, exchanging the position of the sheets on the racks from top to bottom and from front to back at the halfway point, or until the biscuits are golden and crisp. Serve hot or warm.

population will have masa. Since masa contains no gluten, these biscuits are especially tender.