Butter the baking pan after you preheat the oven at 350 F.
Take a large bowl and add all-purpose flour, cinnamon powder, baking soda, and cocoa powder to it and mix. Set this mixture aside.
Now, take another bowl and whisk together mashed bananas, chocolate chips, vanilla extract, sugar, butter, and egg. Pour this mixture into the bowl containing the flour mixture.
Slowly mix all these ingredients until they combine into a smooth batter. Spray the baking pan and add the parchment paper to the baking tray. Now, pour the batter into the baking pan. Bake it for around 50-60 minutes.
After baking, keep it aside to cool for about 10 minutes. For the icing, add cocoa powder, melted butter, icing sugar, and boiling water. Stir the mixture and decorate the cake with this chocolate icing.