Sift the flour with the salt and baking powder. Stir in the caster sugar and rub in the butter. Break the egg into a measuring cup and make up to quarter pint with milk. Whisk the egg and milk together. Make a well in the centre of the flour mixture and pour in the liquid (saving a little for brushing the scones). Using a knife, cut and fold to moisten all the mixture until a spongy, non-sticky ball of dough is formed. Turn the dough on to a floured board, and knead for thirty seconds to smooth the underside. Turn the dough over and roll out gently to half-an-inch thickness. Form into one large round cake (for breakfast) or cut into two-inch rounds (for tea). Place on a buttered baking sheet, and brush with the reserved egg and milk. Sprinkle thickly with granulated sugar. Put into an oven set at 500°F, 250°C, gas mark 10, and then immediately turn the oven down to 425°F, 220°C, gas mark 7 for 10–12 minutes until the scones are puffy and a rich brown. Serve slightly warm, spread with cool butter.