2 x jumbo chocolate Swiss rolls (not the chocolate-dipped ones) or 4 normal-sized ones
150ml/5fl oz alcohol of your choice, such as rum, sherry or limoncello (optional)
2-3 litres/3½-5 pints ice cream (chocolate or vanilla or both), softened but still frozen (no warmer than 12C/54F)
Line the inside of a medium glass bowl or 1.2 litre/2 pint pudding basin with clingfilm (make sure there is plenty of extra clingfilm hanging over the sides).
Cut the Swiss rolls into 1cm/½in slices. Place one slice in the middle on the bowl in the base, then place another next to it. Continue until the whole inside of the bowl is lined. Squash the Swiss roll slices slightly together so that there are no gaps between them. When you reach the top of the bowl, cut slices in half if a whole one does not fit.
Use a pastry brush to dab the sponge with the alcohol. Spoon the ice cream into the lined bowl, squidging it down so that it is completely full. Now place the cake in the freezer and leave overnight or at least until the ice cream is completely firm.
Once the ice cream is firm, carefully remove the cake from the bowl using the clingfilm to help you. Ease it very gently out of the bowl with the clingfilm. If it is too hard to remove, warm the bowl up a little in your hands or by carefully running a flat blunt knife between the bowl and the clingfilm. When the cake is loose in the bowl and feels as if you could pull it out easily, place a large serving plate on top of the bowl and turn the bowl upside down, so that the plate is the right way up. Carefully remove the bowl and peel off the clingfilm.