Mash bananas in one bowl but leave some larger chunks for texture. Add other wet ingredients; egg, sugar, mayo and vanilla. Incorporate together.
In a separate bowl mix together dry ingredients. Combine the two bowls slowly to get a batter. If you want to add crushed nuts or chocolate chips fold them in now.
Spray loaf pan with non stick spray or line with parchment paper. Preheat oven to 350 F. Pour batter into pan and bake for 50-60 minutes or until toothpick in center comes out mostly clean with moist crumbs attached. Do not overbake. Allow to cool for 10 minutes in pan, then loosen slides, slide out and cool on cooling rack rest of the way.
If you'd rather bake muffins fill liners 3/4 of the way full and bake 18-22 minutes or until middle bounces back when touched gently. Remove from pan and transfer to cooling rack immediately.
Notes
Cake Mix Banana Bread no Butter
If you wanted to start with a box of dry mix I would suggest the Spice version as it will give you as close to that flavor you want as possible. Alternatively use yellow. Mash 5 bananas brown + 3 eggs + 1/3 cup of mayonnaise or Greek yogurt. Combine in your 1 box of cake mix for the batter and follow baking directions below.