Sift together the flour, salt, and cornstarch in a large mixing bowl.
Add the cold, cubed, unsalted butter to the flour mixture. Smear the pieces into the flour mix with your fingers and thumbs until all of the butter is smeared and coated in flour.
Add the buttermilk and gently combine the dough with a rubber spatula until a shaggy dough forms, about 1 minute.
After a shaggy dough forms, gently fold the dough over itself and knead several times. Repeat this process 1 to 2 times until the dough just starts to become smooth. This should only take about a minute or two. (The dough should be fairly smooth, but you should still see individual pieces of butter dotted through the dough.)
Turn the dough out onto a lightly floured work surface and gently roll it into a flat disc that is about ¾ to 1-inch thick.
Cut the dough into individual biscuits, then transfer to a greased baking sheet. (Use an egg ring mold or the top of a pint glass as a biscuit cutter).
Brush the tops of the biscuits with melted butter. Bake for 20 to 25-minutes until the biscuits just begin to brown.
Remove the biscuits from the oven and brush with additional melted butter. Broil biscuits on low for an additional 2 to 3-minutes until the tops turn golden brown.
Country Style Sausage Gravy
While the biscuits are in the oven, heat a large, heavy bottom skillet over medium high heat. Add sausage and break into bite sized pieces. Cook for 7 to 10-minutes until browned and cooked through.
Add the flour, fresh cracked black pepper, rubbed sage, and cayenne, stirring to incorporate. Cook for 1 to 2 minutes until the raw flour smell dissipates.
Slowly add the milk while whisking constantly to deglaze the pan. Bring to a simmer, then reduce heat to medium-low and simmer for 3 to 5-minutes until the gravy begins to thicken slightly.
Taste and adjust seasoning for salt and pepper.
Biscuits and Gravy Assembly
Split biscuits while warm, and spoon as much gravy as you want over top.
Notes
The gravy will thicken up significantly as it begins to cool. Add a splash of milk and stir to loosen it back up if needed. Taste and adjust for salt. This is also helpful when re-heating leftovers!See my Buttermilk Biscuit Process Gallery in my post for Buttermilk Biscuits (linked above) for visual reference if needed.