Heat the oven to 190C/fan 170C/gas 5. Tip the flour into a mixing bowl with baking powder and a pinch of salt. Add the butter, rub everything together with your fingers to make a reasonably fine crumbed mixture, then stir in the sugar.
Make a well in the middle of the flour mixture, then tip in the buttermilk and egg. Gently work the mixture together until it forms a soft, sticky, dough.
On a lightly floured surface, knead the mixture a couple of times and mould it into an 18cm round.
Put the dough on a lightly greased baking sheet. Bake for 30-35 mins until golden and risen and a skewer comes out clean when inserted into the middle. Leave to cool.
For the filling, lightly mash the blackberries with the sugar. Split the shortcake in two and spread the bottom half with the clotted cream. Spoon the mashed berries over the cream, then top with the shortcake. Dust with icing sugar and serve in slices.