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 ab
Note: masa is NOT the same thing as cornmeal. Stores catering to Hispanic population will have masa. Since masa contains no gluten, these biscuits are especially tender.