Preheat the oven to 180 degree C. Grease the baking dish with oil and sprinkle flour all over it. Coat the flour on all sides and tap off the excess flour.
Sift the flour, baking powder, cinnamon powder and salt into a bowl and keep aside.
Peel and mash the bananas and keep aside.
In another bowl mix the oil and powdered sugar. Beat the eggs well and mix in the vanilla essence.
Add the flour to the oil and sugar mixture.
Now fold in the eggs and mashed bananas. Gently fold in along with the milk. (Keep aside a tsp of milk)
Add in the chopped walnuts. Mix gently. Dissolve the baking soda in 1 tsp of milk and combine into the batter
Pour the batter into the greased dish and bake for 35-40 minutes. Insert a toothpick to check whether the cake is done.
Remove from the oven and cool for about 10 minutes. Invert it into a steel rack to let it cool down completely.