Heat the oven to 200C/180C fan and line a baking tray with baking paper.
Rub the butter into the flour with your fingertips until the mixture looks like rough breadcrumbs, then stir in the sugar, mixed spice and dried fruit.
Add the beaten egg and enough milk to bring it together into a stiff, sticky dough. Do not overmix and do not panic that it looks rough, that is the whole point of a rock cake.
Spoon 12 rough heaps onto the tray, leaving room between them as they spread a little. They are supposed to look rocky and uneven, so do not smooth them down. Sprinkle each one with a pinch of extra sugar.
Bake for 15-18 minutes until golden brown and firm to the touch.
Cool on the tray for 5 minutes, then move to a wire rack. Eat warm or cold, split and buttered, within 3 days.