Prepare and bake cake according to directions on box. Bake in 2-8in. round cake pans. Remove from pans and cool. Poke holes in both cakes.
Mix jello and boiling water until dissolved. Pour over both cakes. Refrigirate 3 hrs.
Add pudding mix and milk and stir for 2 mins. Fold in cool whip.
Dip cake pans in warm water to help unmold. Spread about 1 cup of pudding mixture over cakes and stack on cake on top of the other. Frost with remaining pudding mixture. Drizzle chocolate on top and sprinkle with chocolate shavings and banana slices.