You need over ripe bananas, I buy them from the store when they have them or I get them from friends and put them in the freezer until I am ready to make banana cake or bread.
I use two boxes of white cake mix and about three big or four medium bananas. I mash the bananas and then add to the measuring cup then add water to the measurement the boxes call for, and then add all the other ingredients and mix.
I bake everything at 325 degrees (habit from working at a bakery that we baked everything at one temperature) and I dont use the time on the box I bake when it just starts to bounce back or tested comes clean then let cool for a while and then wrap in saran and then chill. This holds the moisture in the cake.
I hope you enjoy this.