Line a 8 inch cake pan with plastic wrap and fill with 2 cups blackberries.  If you berries are very tender, freeze beforehand, this way they will hold their shape a little better.

In a food processor, pulse flour, salt, baking powder, baking soda and sugar and lemon zest.  Pulse in cold butter until mix resembles coarse sand.  In a small bowl mix milk and sourdough starter (or sour cream or yogurt) together.  Add starter mixture to the food processor and pulse until it just forms a ball (just a few times) adding a little more milk (only if necessary).  Dough should be heavy and thick.  Don’t over work.

Spread the dough over the blackberries and press down gently, into all the corners.  Cover with plastic wrap and place in the freezer overnight, or for 4 hours.

Preheat oven to 400°F.  Remove from freezer, remove saran, invert on cutting board.  Let sit 15-20 minutes or longer until thawed enough to cut.  Cut into 8 equal size pie shape wedges.  Brush with beaten egg (optional).  Place on parchment lined baking sheet, and bake for 18-20 minutes, until golden brown.

While baking, make glaze.  Stir sugar into lemon juice in a small pot on the stove until dissolved.  Whisk in butter (optional).  Set aside.  Drizzle over warm scones.