Slice 3 lemons very thinly, put them into the very hot syrup made with the sugar and the water. Cook gently for about 1 hour over a very low heat until the slices are transparent but not broken. Oil the pie pan, arrange the cooled down lemon slices on the bottom and on all sides. Boil the milk, pour onto the egg yolks which were whisked with the sugar and the vanilla sugar, heat again, cook without boiling until the cream coats the spoon. Remove from the fire, add the gelatin which has been soaked in cold water, softened and drained, add the 2 lemons peels. Stir until the gelatin melts. Allow to cool, stirring from time to time. Whip the cream without adding any sugar, pour 3/4 of the cream in the completely cooled down mousse. Keep the remaining cream aside to decorate the cake. Cut the Genoese sponge into 2 disks. Moisten one of the disks with half of the syrup in which the lemons were cooked, then dice the disk. Pour half of the mousse into the mold, cover with the moistened Genoese dice then with the remaining mousse. Allow to thicken for a good hour in the refrigerator. Then moisten the second disk with the remaining syrup and put it onto the cooled down cake, pressing gently. Turn out onto the serving dish. Decorate with the remaining cream, whipped again with 1 1/3 ounces sugar.
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For a 9-inch pie pan:
1 ready-made Genoese sponge with the same dimensions
