Combine flour, baking powder, and sourdough starter in a large bowl.
With a grater, grate cold butter into flour mixture. Mix together until it resembles coarse crumbles.
Add milk. The milk measurement will vary depending on the flour you use.
Mix together until dough is just combined. It should be tacky but not sticky. Do no over knead.
Turn out onto a very lightly floured surface, fold dough over onto itself several times, pat into 1″ to 2″ thickness and cut out biscuits.
Place in a buttered cast iron skillet, biscuit sides touching one another, and cook at 425 for 12-15 minutes.