Take the ripe bananas in a mixing bowl and mash well.
Add cinnamon powder and mix well.
Add baking powder and baking soda and mix well.
Coarsely chop the walnuts and add to the mixture.
Add whole wheat flour.
Mix well without any lumps to form a thick mixture.
Add milk little by little and mix the flour mixture with a spatula to get the desired consistency.
The flour mixture is ready to bake.
Grease the baking tray with butter.
Preheat the oven to 300 degrees Fahrenheit and bake it for 35 mins or until the toothpick comes out clean.
When the cake is baked perfectly, remove from the oven and cool it completely.