Preheat your oven to 190°C (375°F/Gas 5). Mix the butter into the flour with your hands or in a blender. Add the sugar, salt and baking powder and then mix in the milk and vinegar until you have a smooth soft dough. Grease a large baking tray with butter and flour.
Roughly divide the dough into three portions. Wet your hands with a little cold water and quickly roll long dough sausages, each about 30 cm (12 inches) long. Your tray needs to be big enough to accommodate them — mine is 34 cm (14 inches) by 25 cm (10 inches) and looks very big when I put the mix in. Arrange the dough rolls parallel on the tray with a few extra dots of butter between them.
Bake for 45 minutes–1 hour until the tops are golden and crusty. Remove from the oven and turn the heat down to 150°C (300°F/Gas 2). The dough rolls will have joined together but their outlines will still be visible. Cut down their lengths to separate them, then cut each roll into 3 cm (1¼ inch) pieces. Don’t touch them for now, leave them in their tray to cool a bit and make them more manageable. Then lift them up and break them in half through their middles so that they look rustic and imperfect. If you find it easier, begin with a knife, chopping through a little and then breaking them apart with your hands.
Return them to the baking tray (lay them on their sides) and the oven for about 30 minutes on each side to dry out a bit. They should be not too toasted, but crumbly and firm. Let them cool completely before storing in a closed container or paper bag, where they will keep well for up to five days.