Preheat oven to 350 degrees and butter an 8 or 9 inch square pan.
In a small bowl whisk together dry ingredients.
In a food processor process the banana until smooth. Add butter and sugar and continue processing until smooth. Next add the egg and process. Finally add the sour cream and vanilla and process until combined.
Add dry ingredients and process only until combined approximately 5 seconds.
Stir with a spatula by hand if needed and pour into the prepared pan. Bake for 30-35 minutes until a toothpick comes out clean.
While the cake is baking you can prepare the frosting. Cream cheese and butter must be at room temperature.
In a large bowl beat together cream cheese and butter until there are no lumps. Slowly add in the icing sugar, then vanilla and a pinch of salt.
Once cake has cooled spread the icing over the cake in the pan, cover with plastic wrap and refrigerate until you are ready to serve. Cut into 16 pieces.