Preheat oven to 350F / 180C.
Grease a 9" cake pan. This size is actually for the recipe which I have halved here so you can only make a thin single layered cake if you use a 9" pan. Try a smaller pan if you want to frost the cake in layers.
Put the flour, baking powder, salt, and soda into the food processor and whiz them together for about 15-20 seconds.
Tip into a bowl and set aside. If mixing by hand, just put everything into a bowl and blend together using a whisk or a fork.
Chop the bananas into the food processor next and whiz into a pulp. Make sure the banana is really ripe. Again, you can do this step by hand using a fork or potato masher, in a separate bowl.
Add the sugar and egg to the mashed banana and whiz for another minute.
Then add the butter and whiz for another minute until the mixture is thick and creamy. (Cream together for about 5 mins if doing by hand).
Next, add the sour cream (or buttermilk) and vanilla extract and whiz in quick pulses until just blended. Fold together if doing by hand.
At this stage, you can either tip the flour mixture into the food processor or fold together in a separate bowl. My chopper bowl is not that big so I did this in a bowl. Add the dry ingredients to the wet ones and fold until just blended. Don't overdo the mixing part.
Scoop into greased tray and smooth out the top layer evenly.
Bake for about 40 mins or until the top turns golden brown and the cake passes the skewer test.
Remove and let cool completely before frosting.
For the frosting
Blend the cream cheese and butter together until smooth
Add the icing sugar and vanilla and process again until completely smooth and creamy - about 3-5 minutes
Refrigerate until needed.
Let the cake cool completely before frosting