For the banana mousse slices, line a loaf pan with plastic wrap and drizzle the bottom with lemon juice. Peel the bananas and cut into slices about 1 cm thick.
Bring whipping cream to a boil and dissolve cooking chocolate in it. Allow to cool and spread over the bananas. Refrigerate.
Heat the white chocolate in a water bath. Mix lemon juice, rum and the remaining bananas. Beat egg whites with granulated sugar until stiff and fold into mixture.
Soak gelatine in cold water, squeeze out and dissolve in 1 tablespoon hot water and stir into the chocolate mixture. Finally, fold in the whipped cream.
Pour this mixture into the loaf pan and cover with the sponge cake layer. Chill the cake overnight in the refrigerator.
Turn out of the mold, remove the foil and brush the surface with cake jelly. Cut the banana mousse slices and serve.