In a medium mixing bowl place butter and flour. Using a pastry cutter, cut butter into the flour until it is pee-sized.
Add in sourdough starter and milk.
Cover bowl with towel and leave at room temperature for 8 hours.
Preheat oven to 425
Remove towel from dough and sprinkle on salt, baking powder, and soda. Fold in dry ingredients without overhanding dough.
Flour a cutting surface. Place dough on it. Using a rolling pin roll to 1" thick rectangle. Cut with a knife into 12 squares. Place on ungreased baking tray.
Bake 8-10 minutes or until golden brown. Place on cooling rack.
Recipe by Grain Mill Wagon at https://www.grainmillwagon.com/sourdough-butter-biscuits/