Preheat the oven to 450°F, and line a large baking sheet with parchment paper. Set aside

In a large bowl, combine flour, baking powder, salt and baking soda. Use a pastry cutter or your hands to incorporate the butter until it resembles a coarse cornmeal.

Add the sourdough starter and ¼ cup of milk to the flour mixture. Use a rubber spatula to mix until a dough begins to form. If the dough is too dry, add the additional milk a little at a time until it comes together to form a cohesive dough.

Turn the dough out onto a lightly floured surface and knead a couple of times. Form the dough into a 1-inch thick rectangle or circle, and use a biscuit cutter to cut out the biscuits.

Transfer the biscuits to the prepared baking sheet. Bake for 14-18 minutes or until golden brown. 

Serve warm. See post for storage options.