Mix the buttermilk and the sourdough starter and let it sit on the counter until it has come to room temperature.
Preheat oven to 425.
Combine the flour, salt, baking soda and baking powder. Mix well. Cut the cold butter into the dry ingredients using a pastry blender, until the mixture resembles cornmeal.
Combine the buttermilk/starter into the dry ingredients until a soft dough forms and it comes away from the sides of the bowl.
Turn the dough onto a floured surface and knead gently for 30 seconds. Do not over work the mixture (you want to have small pebbles of butter showing.
With a floured rolling pin, roll out to 1/2" thick. Cut biscuits and place on a lightly greased baking sheet with sides touching. At this point, you can brush the biscuits with melted butter if desired.
Bake in the center of the oven at 425 for approx. 15 minutes, or until lightly golden brown.