Bananas ripen quickly on the counter, so keeping them on hand means periodically dealing with a few that are past their prime. The good news is that overripe bananas are the richest-flavored, and they work very well in cakes and muffins. One of the simplest ways to conjure up a cake in a hurry is to combine your leftover bananas with a white cake mix. The bananas replace the liquid and part of the fat, and you can add nuts or other ingredients as desired to make the cake your own.
Mash 3 to 4 browned, overripe bananas with a fork, until you have a total of 1 1/2 cups.
Pour your white cake mix into your mixer's bowl with the full three eggs the mix calls for. Cut back the oil to 1/3 cup, and add your bananas.
Mix until all the ingredients come together into a thick batter, then add a half-cup of coarsely chopped walnuts, pecans or other nuts as desired. Stir the nuts into the batter, but do not overmix.
Line two loaf pans with parchment paper, or spray and flour a pair of 9-inch round cake pans. Spoon the batter into your pans and spread it with a spatula.
Bake in a preheated oven at 350 degrees Fahrenheit. Cakes baked in round pans require 30 to 35 minutes of baking time, while those baked in loaf pans need 15 to 20 minutes longer because of their greater depth. Insert a toothpick into the middle of the cake to test its doneness. If it comes out clean, or with just a few moist crumbs adhering to it, the cake is done.
Cool your cakes in their pans for 10 to 15 minutes, then turn them out onto a wire rack to cool completely.
Toast the nuts ahead of time in a dry skillet to improve and freshen their flavor.
The same basic technique can be used with other varieties of cake mix. Yellow cake has a richer color, while spice cake or butter pecan mix give the finished version a richer flavor. A chocolate cake mix still tastes of chocolate, but with a strong and pleasant banana undertone.
For a more virtuous cake, omit the oil and eggs and increase your bananas to 2 1/2 cups. This version makes a denser cake with a brownie-like texture, and it should not be baked in loaf pans.