Preheat oven to 400 degrees F. Line a baking sheet with parchment or a silicone baking mat.

In a medium bowl combine flour, sugar, baking powder, salt, and stir to combine.

Add the butter and cut in with a pastry cutter It should look like sand with a few larger (pea-sized) pieces of butter.

In a small bowl whisk the egg, sour cream, and vanilla together. Pour this mixture into the flour mixture and stir just until combined. Don't overmix, the dough will be wet. Fold in the frozen berries.

Turn dough onto a lightly floured surface and work into an 8" round. If it's too wet and sticky, add a bit more flour.

Using a bench scraper, or large chef's knife cut into 8 wedges. Carefully transfer wedges to prepared baking sheet with a spatula, spacing them about 2" apart.

Place milk in a shallow bowl and brush each scone with milk and sprinkle with sanding sugar.

Bake 18 - 20 minutes, checking at 18 to make sure they don't over bake.

Cool for 10 minutes on the baking sheet before removing them to a cooling rack. Serve warm or at room temperature.