Gently heat ¼ cup buttermilk in small bowl in microwave until between 105 and 110F. Add yeast and 1 teaspoon sugar and mix with a fork until both are dissolved. Let sit until mixture is very foamy, about 5-6 minutes.
In a large bowl, whisk together flour, salt, sugar, baking soda, and baking powder.
Using a pastry cutter, cut shortening into flour mixture until pea-sized crumbs develop.
Pour in yeast mixture and 1 cup buttermilk and stir with spatula or wooden spoon until a shaggy dough develops.
Transfer biscuit dough onto a floured work surface and knead gently just to combine, but do not overmix.
Cover with a clean kitchen towel and place in warm spot (around 85-90F) until dough has doubled in size, about 1 hour.
Remove dough and roll into a ½ inch thick round using a rolling pin. Cut into 2 ½ inch round cutter (do not twist when cutting biscuits) and place biscuits in greased cast iron skillet.
Cover biscuits with a kitchen towel and place back in warm spot until almost doubled in size, about 30-40 minutes.
Place biscuits in freezer for 25-30 minutes to rechill the shortening.
Meanwhile preheat the oven to 375F. Place oven rack in middle position.
Brush biscuits with 2 tablespoons buttermilk and sprinkle with flaky sea salt, then bake until lightly golden brown, about 25-28 minutes.