Preheat oven to 350 degrees Fahrenheit (180 degrees centigrade) and place rack in centre of oven.
Butter or spray two 9 x 2 inch cake pans.
Toast the walnuts for about 8 mins or until lightly browned and fragrant. Cool and chop coarsely.
In a separate bowl whisk together the flour, baking soda, baking powder, salt, ground cinnamon, and cocoa powder. Set aside.
Beat the eggs until frothy. Gradually add the sugar and beat until the batter is thick (about 3 to 4 mins).
Add the oil in a steady stream and beat in the vanilla extract.
Add the flour mixture and beat until incorporated.
Fold in the pieces of banana and chopped walnuts.
Evenly divide the batter between the two prepared cake pans and bake 25 to 30 mins or until a toothpick inserted in the centre comes out clean.
Remove from oven and cool on a wire rack.
After about 5 -10 mins invert the cakes onto the wire rack and cool completely before icing.
To assemble: place one cake, top side down, onto your serving plate. Spread with about a third of the icing cream. Gently place the other cake, top side down, onto the icing cream, and spread the rest of the icing cream over the top and sides of the cake.