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. Alternatively, place flour and frozen butter in blender; pulse ~16 times, until butter is fragmented to no larger than pea-size.
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.
*All purpose flour will work but won't rise as well or be as fluffy as a lower protein flour. Cake flour or White Lily flour are ideal, or remove 1 T. of flour from the cup and replace with 1 T of cornstarch, then sift thoroughly. Or just use the AP, it'll work. I use 2-to-1 cake flour/AP flour.