Preheat the oven to 120°C (235°F/Gas ½) and line a baking tray with baking paper. Slice right across the cake to make strips 1.5 cm (⅝ inch) thick. Cut these into 2 or 3 long lengths and lay on their sides on the lined tray. Bake until dry and golden, about 40 minutes. Remove from the oven and cool on the tray. Store in a biscuit tin where they will keep for many weeks.