2) Sift the flour, bicarbonate of soda, spices, salt and 200g of the sugar in a medium bowl. Stir in the chopped crystallised ginger.
3) Lightly beat the egg in a small bowl and add to the cooled butter-treacle mixture. Fold the flour into the mixture and combine well. Wrap the dough in cling film and chill for 20 to 25 minutes.
4) Preheat the oven to 190C/Gas 5. Put the remaining 50g of sugar in a shallow bowl and set aside. Take 2 tbsp of the dough and roll it into a ball, then into the sugar. Repeat with the rest of the dough.
5) Bake the biscuits until flat and just starting to dry out on the edges, about 10 to 14 minutes. Bake in batches of 8 on a baking tray lined with a silicone baking mat or aluminium foil.
6) Remove to a cooling rack. The biscuits can be cooked ahead of time, cooled and stored tightly covered.
7) Cut the packaging from the ice cream and cut the ice cream into 2.5cm thick rounds. Put 1 biscuit face down on a 20cm square of greaseproof paper and top with the ice cream, matching the diameter of the biscuit as closely as you can, trim if needed. Top with another biscuit, face up and wrap the greaseproof paper around the stack, and then wrap tightly with a piece of cling film.
8) Repeat, placing the finished stackers into the freezer as they are completed. Let them freeze for 30 minutes and remove wrapping to serve.