1 egg, beaten
Preheat the oven to 220°C/425°F/Gas mark 7.
Sift the flour, salt and baking powder together in a large bowl. Rub in the butter until the mixture resembles breadcrumbs. Stir in the sugar and frozen blackberries. Stir in the buttermilk to form a firm dough.
Knead the dough very lightly. Roll out on a floured surface to 2.5cm/1in thick. Using a 6.5cm/2 1⁄2in cutter, stamp into rounds, being careful not to cut through the blackberries. Knead and re-roll as necessary.
Put the scones on a greased baking tray, and brush the tops with beaten egg. Bake in the oven for about 10 minutes until well risen and golden brown. Serve while still hot.