Preheat a standard cast iron skillet on the stove or in the oven at 425F

Add the flour, salt, sugar, and baking powder to a mixing bowl or bowl of a food processor and mix to combine.  

Grate/cut the cold butter into the dry ingredients.  

Add the sourdough starter, egg, and milk - then stir until combined with the butter and flour mixture, but do not overmix or knead - the dough should be shaggy.  

Turn the biscuits out onto a lightly floured surface and fold the dough over onto itself until the dough sticks together in one piece.  

Then, flatten the biscuit dough with your hand or a rolling pin into a thick rectangle and tri-fold the dough.  

Then flatten it out and tri-fold it again.  Repeat this flattening and folding process 5-6 times.  

Never knead the dough, just flatten and fold it -this technique is what will give you flaky sourdough biscuits with lots of layers. 

Flatten or roll the biscuit dough out into a thick rectangle and use a biscuit cutter or bench scraper to cut the biscuits.

Add 2 tablespoons of butter to the hot skillet and return it to the preheated oven for a few minutes until the butter is browned.  

Once the butter is browned, make sure to spread it around to cover the entire surface of the skillet, then add the cut biscuits and bake at 425 for 15-18 minutes until the tops just start to turn golden brown.  Serve immediately or store for later.