Preheat the oven to 425 degrees Fahrenheit and prepare a parchment-lined baking sheet.
Add all dry ingredients and very cold butter to the food processor bowl.
Pulse until the mixture is crumbly and the butter pieces are pea-sized or smaller.
If you're not using a food processor, then add the dry ingredients to a large bowl, add the cold butter in small cubes, then use a pastry blender to incorporate the butter.
Add sourdough starter and vanilla, then pulse until a ball is formed. If too dry, add 1-2 tablespoons of milk (more, if needed).
Turn the dough out onto a clean workspace. Gently fold in the blueberries. Working on parchment paper makes this part easy. Shape the ball of dough into a square, then cut into triangles.
Transfer to a prepared baking sheet. Optional: Prepare egg wash by whisking one egg with one tablespoon of water. Brush over the tops of the scones.
Bake for 15-20 minutes or until golden. Allow to cool.
Lemon Glaze
Create lemon glaze by whisking powdered sugar and lemon juice. Add more lemon juice if needed.
Drizzle over cooled scones.
Notes
Freeze the butter for 30 minutes before preparing the dough so that it doesn't soften while being prepared.
Handle the dough as little as possible.
Use sourdough discard for a stronger tangy flavor, or active starter for a much milder tone.
Frozen blueberries need to be folded in pretty quickly to avoid a blue or purple dough.
As an option, you can drizzle a glaze over the scones after baking.