Pre-heat the oven to 400°F (207°C). Combine the flour, sugar, baking powder, and salt. Add in the butter until the mixture resembles coarse crumbs, and then follow by the egg and vanilla. Add in the milk slowly.
If the dough becomes too sticky, stop adding the milk, instead add a little more flour to form the dough. Stir until the dry ingredients are moistened. Add in the blueberries.
Roll out the dough on a floured surface to 3/4-inch thickness. Using a 2-inch round cookie cutter, cut out the scones. Using your hands to smooth out the surface of the scones to make them slightly rounder on the edges.
Gather the scraps; re-roll and cut out additional scones. Place the scones on prepared baking sheets, spacing apart.
Brush egg yolk over the tops of the scones. Bake scones until golden brown, about 15-20 minutes. Serve warm.
Notes
If the dough is too sticky, add more flour to the dough.