In a mixing bowl add the banana and mash them with fork or you can use electric beater.
In the banana mixture add buttermilk, sugar, oil, vanilla extract and mix well.
In a mixing bowl sieve the flour, baking powder, baking soda, cinnamon powder for twice and mix well.
Add the dry ingredients into the wet ingredients and gently fold to make a smooth batter.
Add the flour coated walnut into the batter and give a quick stir. Do not over mix the batter.
Grease a cake pan with oil and preheat oven @180° for 10 mins.
Pour the batter into the greased pan and tap 2-3 times to release the air bubbles inside. Garnish with chopped walnut and banana slices.
Bake @180° for 35-40 mins until a toothpick inserted comes out clean.
Remove the cake from oven and let it cool down. Invert and slice after the cake is cooled down completely.