Preheat oven to 350 degrees. Spray a 9x13 baking pan with nonstick baking spray.
Add the peeled bananas to a large bowl and mash with a fork.
Add oil (or 1 stick of butter melted), eggs, and vanilla. Using a whisk, mix until combined.
Add in the yellow cake mix until whisk until all ingredients are blended together.
Pour the batter in the prepared pan. Spread the batter evenly in the pan.
Place in the oven and bake for 25 to 30 minutes or until a toothpick inserted in the middle comes out clean or with moist crumbs.
Cool completely before frosting.
Frosting
In a large mixing bowl, add in the butter and cocoa powder. Beat with a hand mixer until creamy.
Add in the powdered sugar, vanilla, and heavy cream and continue to beat until combined. You may add additional heavy cream 1 tablespoon at a time to get the desired consistency of frosting.
Spread the frosting on top of the cake with an offset spatula.