For the Biscuits

1. Preheat the oven to 425 degrees F. Adjust the rack to the middle position, and line a large baking sheet with parchment.
2. Put the flour, cornstarch, baking powder, baking soda, sugar, and salt into a large mixing bowl and whisk very well to thoroughly combine (alternatively, you can add them to the bowl of a food processor). Add the butter and, using a fork or a pastry blender, work the butter into the flour until the mixture resembles coarse sand with some pea-sized clumps remaining (if using a food processor, just pulse everything together until it resembles coarse sand). 
3. Add the cold buttermilk and, using a spoon/spatula now, combine the mixture until you have a shaggy mass. If it feels pretty dry, add another splash of buttermilk until it comes together for you. Just take care not to overmix. 
4. Turn out the dough onto a floured work surface (you can dust the top of the dough with flour as well, if it’s really sticky). Pat the dough into a 3/4-inch-thick rectangle and cut it evenly into thirds. Stack the thirds on top of one another, and pat the dough down again into a 3/4-inch-thick rectangle.
5. Repeat the last step by cutting the dough into thirds, stacking those on top of one another, and patting down into a final thickness of about 1 inch. Using a knife, cut the biscuits into 12 square biscuits (you could do 8 larger squares, if you prefer). Put the biscuits onto the prepared baking sheet and brush each with a little buttermilk. Sprinkle them with a little salt, if you like. 
6. Bake until very golden on their tops and bottoms, and cooked through; about 12 to 15 minutes.  

For the Buttermilk Gravy

1. I like to make the gravy while the biscuits bake and cool. Set a large skillet over medium heat and add the sausage, scallion whites, and poultry seasoning. Break up the sausage as much as you can, and allow it to fully cook, getting nice and brown; takes about 5 minutes. 
2. Sprinkle the flour over the sausage (no need to drain the pan – we need the drippings to help toast the flour), and let it cook (stirring) for about a minute.
3. Add the buttermilk, stirring as you do to break up any lumps. Let the gravy come up to a bubble, and then reduce the heat to low. It only takes a few minutes to thicken and is ready to serve right away. If it seems too thick to you, you can add more buttermilk or regular milk (in ½-cup increments) until the consistency is to your liking. You’re looking for something that is spoonable – that will pour down easily over the biscuits, but not soupy or thin/liquidy. 
4. To serve these biscuits and gravy, I like to split a biscuit in half and lay the halves on a plate, cut sides facing up. I spoon a generous amount of gravy down over top and sprinkle on the reserved scallion greens.