Cut butter into small pieces and return to fridge (or place in freezer for a few minutes).
Combine dry ingredients and mix thoroughly.
Fold butter into flour; pinch larger pieces into dough until there's nothing bigger than pea-sized.
Mix in sourdough starter. Fold over repeatedly, turning 1/4 turn in between folds.
Roll out to 3/4" thick and cut into biscuits. Place biscuits into ungreased baking pan (a round metal cake pan works well but anything will do).
Bake at 425 for 12-15 minutes, until tops begin to brown.
*Other all purpose flour will work but won't rise as well or be as fluffy; White Lily is low-protein--a reasonable substitute is either 50/50 all purpose and cake flours, or remove 1 T. of flour from the cup and replace with 1 T of cornstarch, then sift thoroughly.