Pre-heat oven to 425 degrees and line a rimmed baking sheet with parchment paper.
In a large bowl, whisk together the flour, cornstarch, salt, sugar, baking powder, and baking soda.
Using box grater, grate the butter directly into the flour mixture. Stir together until the butter is dispersed and fully coated by the flour.
Make a well in the bowl and pour in the buttermilk, reserving 1 tablespoon.
Stir together until just combined, the dough will still be a bit shaggy.
Dust the countertop with a couple of tablespoons of flour and pour the dough onto the countrertop.
Using a bench scraper, fold the dough on top of each other until it forms a cohesive mass.
Then dust a rolling pin with flour and roll out the dough about 8 x 4 inches. Fold the dough, one third at a time, like creating a letter. Repeat 3 to 4 more times and then roll into a 8 x 4 rectangle.
Trim the edges using a bench scraper or sharp knife.
Then cut into 6 equal pieces.
NOTE: Save the scraps and make little mini-bitesize biscuits to bake later.
Brush the biscuits with some of the remaining 1 tablespoon of buttermilk, sprinkle with sea salt and bake for 10 to 12 minutes until the biscuits are golden brown.
Remove from oven, brush with the melted butter and serve immediately.