Separate eggs. Using a mixer, beat yolks with mashed banana and oil until light and fluffy. Add sponge cake mix gradually and beat very thoroughly. Beat egg whites until stiff. Fold egg whites and chopped nuts into batter gently but thoroughly. Pour batter into lightly greased 9 x 13 pan and bake in moderate oven 10-12 minutes or until toothpick comes out clean in center.
Set aside cake and let cool completely.
Meanwhile melt chocolate over double boiler and let cool down partially. When cake is completely cooled, pour chocolate over top of cake - spreading evenly. Place sliced bananas onto melted chocolate evenly. Place cake in freezer for 25 minutes or until chocolate has hardened. Remove from freezer and spread a thick layer of vanilla ice cream over top of cake – refreeze for an additional 40-45 minutes. Remove from freezer and serve with drizzled chocolate sauce.