Add oil+ mashed Banana and mix using electric mixture. Add eggs+ vanilla+ butter milk and mix well for 3 minutes.
Pour batter in greased& battered tube baking tray. Bake in 375F (or 180C) preheated oven for about 40-45 minutes, or till a tooth pick comes out clean when inserted in cake.
Remove from oven. Set in tray 10 minutes. Remove and cool on a rack. Transfer to serving plate, drizzle with chocolate sauce and decorate with whole walnut pieces.
While cake is baking, prepare the sauce: melt butter. Add cocoa and stir with whisk till smooth. Add icing sugar+ milk, stir well till smooth. Add vanilla and stir. If sauce is very thick, add 1-2 tbsp water and stir. Set aside to cool. (You can add or reduce the amount of sugar in the sauce according to your preference).