“This cake highlights the classic flavour combination of chocolate, bananas, and peanut butter, except that no bananas are used—just banana peels! The peels have a surprising amount of banana flavour, not to mention potassium, fibre, and antioxidants. They also help to ensure an incredibly moist cake.” —Lindsay-Jean Hard, Michigan
To make the Banana Peel Cake: Heat the oven to 350°F (175°C).
Line the bottom of 2 8-inch (20 cm) cake pans with parchment paper and spray the sides and bottoms with no-stick cooking spray.
Roughly chop the banana peels. Add the peels and the buttermilk to a tall, narrow container and purée with an immersion blender until completely smooth—you’ll still see black flecks, and that’s fine (a blender or a mini-food processor would do the trick, too).
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and white sugar until pale and fluffy, about 3 to 5 minutes. This can be done by hand with a wooden spoon; it will just take more time. Add the egg yolks one at a time, mixing until incorporated, scraping down the sides of the bowl after each addition. Add the banana peel mixture and the vanilla paste or extract, and mix until well combined. It will look curdled and that is normal.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and sea salt. Add the dry ingredients to the butter mixture and stir gently, just until combined.
Whisk the egg whites in a clean, dry bowl until soft peaks form, either by hand or with the whisk attachment on a hand mixer or immersion blender. If using an electric mixer, start slowly and gradually increase the speed to medium high. The egg whites are ready when you pull out the whisk or beater and a soft peak is formed, and then immediately collapses. Gently fold the stiffened egg whites into the batter and divide the batter evenly between the 2 prepared pans.
Bake for about 20 minutes, rotating the pans 180° halfway through. Insert a toothpick into the centre of each cake and if it comes out dry, the cakes are done. If the toothpick is sticky with wet dough, return the cakes to the oven for 5 or 10 minutes and check them again. Let them cool completely in the pans before transferring to a plate.
To make the Peanut Butter Frosting: In the bowl of a stand mixer or a metal mixing bowl, combine the egg whites, sea salt, and white sugar, and mix to combine. Transfer the mixture to a double boiler and heat until the sugar is completely dissolved and the temperature reaches 180°F (80°C). This can take a while—be patient!
Using the whisk attachment on the stand mixer (or a hand mixer), whip the egg white and sugar mixture on high speed until it doubles in volume and becomes thick and shiny. Continue whipping until the mixture is cool. Check the temperature by placing your hand against the bottom of the bowl.
Lower the speed of the mixer to medium, and add pieces of butter, 2 or 3 at a time, until all of the butter is incorporated.
Beat on high for 1 minute to ensure the butter is completely combined.
Add the vanilla paste or extract and peanut butter, and whip until completely combined, about 1 minute. If the kitchen is warm and the frosting is too soft to work with, transfer it to the fridge to firm up.
Use peels from very ripe bananas—the kind you would use for banana bread. Using peels from eating-ripe-but-not-really-ripe bananas will work, but you’ll get less of a pronounced banana flavour. As always, when eating the typically discarded outer layers of produce, choose organic if possible, and scrub well.
Freezing and thawing the banana peels enables you to soften them without cooking. You can store the peels in an airtight container in the freezer for months, and you’ll be prepared to bake this cake whenever the mood strikes. It will take about 3 hours for the peels to freeze and about 1 hour for them to thaw at room temperature.