Step by step recipe:

1. 1 hour

   The rum-soaked sponge

   Make 500 g Génoise (Genoa sponge) in a 8 x 8 inch (20 x 20 cm) tin or mould.
2. 5 min.
   Cut the cake in half lengthways.
   
   If necessary, trim off any crusty edges to neaten the sides. Sit the pieces on a baking sheet.
   
   Note: when cutting cake like this, a serrated knife makes the job easier.
3. 3 min.
   Prepare the soaking syrup by mixing 150 g Sugar syrup and 30 g rum in a bowl.
4. 2 hours
   Use a brush to soak both pieces of cake generously with the syrup.
   
   Put in the freezer for at least 2 hours.
5. 1 hour

   The ice creams

   Once the soaked cake is fully frozen, cover one with a layer of 400 ml Vanilla ice cream.
   
   Spread quickly, the flatten the top with a palette-knife.
   
   Put back in the freezer for an hour.
6. 5 min.
   After this, add 400 ml blackcurrant sorbet as a second layer and flatten the top as before.
7. 5 min.
   Place the second piece of soaked cake on top, then neaten the sides, adding a little more ice cream to fill any gaps.
   
   The "sandwich" should be a tidy rectangular block in 4 layers: soaked cake base, vanilla ice cream, blackcurrant sorbet and soaked cake lid.
8. 2 hours
   Sit this ice-cream sandwich in an oven-proof dish, cover with plastic film and return to the freezer one last time for at least 2 hours.
   
   The assembled block can be made in advance and kept like this for several days.
9. 2 min.

   Finish the dessert

   About 15 minutes before you intend serving, preheat the oven to 460°F (240°C). Take the ice-cream sandwich in its dish out of the freezer.
10. 5 min.
    Prepare a French-style meringue by beating 3 egg whites to stiff peak stage and folding in 150 g caster sugar at the end.
11. 5 min.
    Cover the ice-cream sandwich completely with an even layer of meringue, using a palette-knife once more, if possible, to smooth the surface.
12. 7 min.
    Put any leftover meringue into a forcing bag with a fluted nozzle and pipe decorations on the top and around the base.
    
    Finish by scattering a few flaked almonds over.
13. 5 min.
    Bake in the very hot oven for about 5 minutes, just enough to lightly brown the meringue and give it a thin crust.
    
    You can also finish the dessert under the grill or with a torch, like for a crème brulée.
14. 5 min.
    Serve a slice to each guest, either on its own or with a little real custard (crème anglaise).