Preheat the oven to 180°C (350°F), Gas Mark 4, and line a baking sheet with baking paper.
Place the butter and sugar in a food processor and blend until well mixed. Add the flour and spice, and blend to just combine. Tip out on to a smooth surface and knead lightly to form a soft dough.
Shape into a 15-cm (6-inch) long, square-sided cylinder and roll in the extra sugar. Flatten the sides slightly, to give the dough a square cross-section. Cut into approximately 16 square slices and place on the prepared baking sheet. Cook in the preheated oven for about 15 minutes, until lightly golden.
Transfer to a wire rack to cool slightly, then serve warm with scoops of toffee ice cream.