Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper. Set aside.

In a large bowl, stir together flour, sugar, baking powder, baking soda, and salt in a large bowl.

Rub the butter into the flour mixture with a pastry blender or your fingers until the mixture resembles coarse breadcrumbs. Toss in the blackberries.

Combine the buttermilk and vanilla extract in a small jug, then pour into the flour mixture. Stir gently until the dough just comes together.

Turn the dough onto the prepared baking sheet and press into a circle that is 1-inch (3 cm) thick and about 7 inches (18 cm) in diameter.

Cut the circle into 8 wedges and push apart so they have room to expand.

Brush the tops with egg wash and bake for 20-22 minutes, or until the tops are golden brown and the scones feel firm when lightly pressed. Let cool completely on a wire rack.

In a small bowl, whisk together the powdered sugar, butter, vanilla bean paste and 1 tablespoon of milk. The glaze should be thick enough to coat the back of a spoon but pourable. If it is too thick, add more milk, a few drops at a time, until the desired consistency is reached.

Drizzle the glaze on the scones and leave undisturbed for 10 minutes, or until the glaze is set.

These are best the day they are made but leftovers can be stored in an airtight container at room temperature for up to two days.