Take four bananas and mash them in a bowl. Then mix the butter, sugar, and eggs together, and add them to the mashed banana. Sieve the flour, baking soda, and salt well, and then add them as well. Stir all the ingredients into a smooth batter. Finally, carefully fold in the walnuts.
Put the batter into a loaf pan lined with baking parchment. Bake it for 60 minutes at 350°F until the top is golden brown and firm. When the banana bread is done, leave it to cool. Then cut it in slices roughly ¼ inch thick and use them to line the bottom of a casserole dish.
Dissolve the instant coffee in cold water, and add a little rum if desired. Use a pastry brush to spread half of this mixture on the banana bread in the casserole dish.
Mix the cream and sugar, then beat it until the mixture is stiff. Cover the banana bread layer with some of the sweetened cream, making sure it's completely covered.
Cover the layer of whipped cream with fresh banana slices, then put another layer of banana bread on top. Apply the rest of the espresso-rum mixture to the banana bread, then add another layer of cream on top and smooth it out flat.
Finally, use a sieve to sprinkle a layer of cocoa powder over the cream, until you can no longer see the cream.