Step 1

Preheat the oven to 425ºF and set an oven rack in the middle position. Line a 13x18-inch baking sheet with parchment paper.

Step 2

In the bowl of a food processor, combine the flour, cornstarch, baking powder, baking soda, sugar and salt. Pulse a few times to mix. (Alternatively, combine the ingredients in a mixing bowl.)

Step 3

Add the butter and pulse until the mixture resembles coarse sand with a few pea-sized clumps of butter intact. (If making by hand, "cut" the butter into the dry ingredients with a pastry cutter, two knifes, or your fingertips.) Transfer the mixture to a mixing bowl.

Step 4

Add the buttermilk and stir with a spoon or rubber spatula until the dough comes together into a shaggy mass. If the dough seems too dry, add a few more tablespoons of buttermilk. Do not over-mix.

Step 5

Turn the dough out onto a lightly floured work surface. Dust the top of the dough with a bit more flour and bring together gently into a loose ball. Pat the dough into a rectangle about 3/4 inch thick.

Step 6

Using a sharp knife, cut the dough into thirds. Stack the pieces on top of one another and pat out into a rectangle about 3/4 in thick again, flouring the surface lightly as needed to prevent the dough from sticking.

Step 7

Cut the dough into thirds again. Stack the pieces on top of one another and pat into a rectangle with a final thickness of about 3/4 inch.

Step 8

Dust the blade of a sharp knife with flour and cut the dough into twelve even squares (they will seem small). Transfer the squares to the prepared baking sheet and bake for 13 to 15 minutes, until the biscuits are lightly golden on top and a rich, golden brown on the bottom. The biscuits are best served warm out of the oven (though a few minutes in the oven will revive slightly older biscuits).