Preheat the oven to 475°F (245°C) and grease a baking sheet or cast-iron skillet.
In a large mixing bowl, whisk together the self-rising flour and kosher salt until evenly combined.
Cut the shortening and the very cold butter into the flour mixture using a pastry cutter or fork until the mixture resembles coarse crumbs with pea-sized pieces of fat.
Slowly pour the buttermilk into the dry ingredients and gently stir until a shaggy dough forms; do not overmix.
Turn the dough onto a lightly floured surface and gently pat or roll it into a 1/2-inch thick round, folding once or twice if needed to create layers.
Use a sharp metal or glass biscuit cutter to cut rounds, pressing straight down without twisting, and place them on the prepared baking sheet or in the skillet so sides touch for taller biscuits.
Brush the tops lightly with melted butter, if using.
Bake in the preheated oven for 10–12 minutes, until the tops are golden brown.
Remove from the oven and serve warm.