With the cake week completed, and one of their number already eliminated from the competition, the 11 remaining home bakers must now prove their skill at making biscuits, a task that can be deceptively difficult, and one that has undone many a confident Bake Off contestant in the past. The opening signature challenge this week is to create 24 biscotti, Italian delicacies that must be baked twice to deliver the perfect crisp crunch.