Chop banana into thinly slices. In a bowl, beat eggs. Crush half of walnuts roughly. (keep the rest for topping)
Preheat the oven to 350 degrees F. Line baking sheet with parchment paper on the baking pan.
In a sauce pan, combine 1/4 cup milk, 1/4 cup water and 4 tablespoons butter. Heat over medium heat. When it boils, turn off the hea, stir until butter is completely melted and then add 1/2 cup flour and 2 tablespoons cocoa powder. Stir well. Then add 2 beaten eggs little by little. Mix well with spatula until smooth. Put the batter into a pastry bag.
Pip the batter on the parchment paper drawing circle (6-7 inch diameter) and bake for 30-35 minutes. Let cool.
In the mean time, make cocoa cream. In a bowl, combine 2 tablespoons cocoa powder, 1 tablespoon Splenda and 1.5 tablespoons flour. Mix well. Add 1 cup milk little by little. Whisk well. Heat in a microwave for 2 minutes. Take the bowl out of the microwave and stir well. Put the bowl back into the microwave and heat for 1 minute. Let cool. (Even if it is not thick enough, it is going to be thick when it gets cool. Do not make it too thick at this time)
When the puff is done, slice it into half crosswise. Spread cocoa cream and then cool whip on the one of puff sheet. Then put half of sliced bananas and crushed walnuts. Put the other puff sheet on top. Place the rest of bananas and walnuts on top ot the puff cake and drizzle caramel topping.