Rub the cold butter into the flour and salt until it looks like coarse breadcrumbs, then stir in enough cold water to bring it together into a firm dough. Wrap and chill for 20 minutes.
Mix the currants, brown sugar, softened butter and mixed spice into a rough paste. A little grated lemon zest is traditional if you have one.
Roll the pastry out on a floured surface to the thickness of a pound coin. Fold it in thirds like a letter, turn it 90 degrees, roll again, and repeat once more. This layering gives the cakes their flaky sides. Roll out finally to about 3 mm thick.
Cut 8 rounds with a 10 cm cutter or use a small saucer as a template. Put a heaped teaspoon of the currant filling in the middle of each, brush the edges with milk, then gather the pastry up and pinch it firmly over the top to seal in the fruit.
Turn them seam side down and flatten gently with the rolling pin until the fruit just starts to show through. Brush with milk, scatter over the demerara sugar and make 3 small slashes across the top of each.
Bake at 200C (180C fan) for 15 to 18 minutes on a lined tray until deeply golden and the sugar is caramelised. Rest on the tray for 5 minutes before lifting, the filling stays molten hot.