Preheat the oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together the milled wheat flour, sugar, baking powder, baking soda, and salt.
Add the cold butter and use a pastry cutter or your fingers to cut it into the flour mixture until it resembles coarse crumbs.
Stir in the sourdough starter, buttermilk or milk, and vanilla extract. Mix until just combined. The dough should be slightly sticky.
Turn the dough onto a floured surface and gently knead it a few times. Pat the dough into a ¾ inch thick rectangle and use a biscuit cutter or a glass to cut it into rounds.
Place the sourdough biscuits on the prepared baking sheet and bake for 10-12 minutes, or until golden brown.