Mix the flour, baking powder and salt in a large bowl. Which together to evenly distribute the baking powder. No one wants a mouthful of baking powder.
Add the cold butter and cut it into the flour with the pastry cutter. You can use your hands to blend the butter by squeezing the flour into each chunk of butter. Be careful not to handle the butter too much.
Stir in the milk until all flour is wet.
Add in the sourdough starter and knead until the mixture is well combined.
Cover with a damp tea towel or cloth and leave at room temperature for 8-10 hours or in the fridge for up to several days. *top of the dough may turn grey from the baking powder, but I find that keeping the cloth on top damp stops this from happening.
Preheat the oven to 450 ℉.
Dump the dough onto a lightly floured surface and knead it 10-15 times. Roll the dough out to about 1.5" thick. Fold in half.
Roll the dough out again until 1.5" thick. fold in half. Repeat this process 4 or more times to create layers in your biscuits. Try not to use your hands because the butter will melt. You want cold chunks of butter to still be visible in your dough.
Roll the dough out to about 1" thick an cut the biscuits out with a biscuit or cookie cutter. Push the cutter straight down into the dough. Do not twist.
Arrange the biscuits in you skillet or on your pan, either touching on all sides, or just beryl separated form each other.
Brush the tops of the biscuits with melted butter and bake for 15 minutes. Biscuits should be golden brown. Continue to bake if the biscuits do not have enough color yet.