Begin by combining the dry ingredients. In a large mixing bowl, whisk together the flour, sugar, salt, and baking powder until evenly blended. Incorporate the cold butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.

In a separate bowl, whisk the egg, buttermilk, and vanilla extract until well integrated. Gently fold the wet mixture and the blueberries into the dry ingredients, mixing just until a cohesive dough forms.

Lightly flour a piece of parchment paper or your countertop, then press the dough into a circular disc approximately 8 inches in diameter and ¾ inch thick. Use a pastry brush to coat the top with additional buttermilk.

Slice the dough into 8 triangular wedges using a sharp knife. Space the wedges apart on a baking sheet lined with parchment paper or a silicone mat, allowing room for expansion during baking. Chill the scones in the refrigerator for 20 minutes.

Preheat the oven to 400°F (200°C) while the dough rests. Bake the scones for 20 to 25 minutes, or until the tops are golden brown. Remove from the oven and let cool slightly before glazing.

Prepare the vanilla glaze by whisking together the powdered sugar, milk (or water), and vanilla extract in a small bowl until smooth. Generously drizzle the glaze over the warm scones.

Serve immediately for optimal freshness or allow to cool completely. Store leftovers in an airtight container at room temperature for up to three days.