1. Preheat the oven to 180 °C.
2. In a bowl, whip the 2 egg yolks and the sugar until the mixture whitens. Add the fl our and mix it in. Delicately incorporate 2 beaten egg-whites. Pour into the rectangular dish, buttered and fl oured, and place in the oven for 25 minutes.
3. Turn out while hot and leave to cool on a grill.
4. When the sweet omelette is cold, cut it in two horizontally. Place the first half in the cooking dish, cover in ice-cream, then add the second half. Press together, cover and place it in the freezer.
5. At the moment of serving, preheat the oven to 140 °C. Whip the 2 remaining egg whites until they are very stiff, incorporating the icing-sugar. Take the cake out of the freezer and decorate it with the egg-whites (using a spoon or an icing-bag).
6. Put the cake in the oven under the grill and leave for 1 minute, until it becomes nicely golden. Serve straight away.