BANANA PEEL CAKE WITH BROWN SUGAR FROSTING

Steps:

• Heat the oven to 350 degrees F.
• To make the cake: Cut the banana peels into 1-inch pieces and place them in a small saucepan with 1 cup of water. Bring to a boil over medium-high heat, then reduce the heat and simmer for 10 minutes. Remove the pan from the heat and allow the mixture to cool slightly, then drain the banana peels, reserving 1/4 cup of the cooking water.
• Meanwhile, butter and flour the sides of two 8-inch round cake pans and line the bottoms with parchment paper. Butter and flour the pans again to coat the paper.
• Transfer the peels and the 1/4 cup of cooking water to a tall, narrow container and puree with an immersion blender until completely smooth (a mini food processor would do the trick, too!).
• Cream together the butter and sugar in a large bowl using an electric mixer (or a wooden spoon for an arm workout) until pale and fluffy, 3 to 5 minutes. Add the egg yolks one at a time, mixing until incorporated, and scraping down the sides of the bowl after each addition. Mix in the banana peel mixture, then stir in the buttermilk until well-combined.
• In a medium-size bowl, whisk together the flour, baking soda, baking powder, and salt. Add the dry ingredients to the bowl with the butter mixture and stir gently, just until combined.
• Put the egg whites in another bowl (make sure it's clean and dry!) and whisk until soft peaks form -- either by hand or with the whisk attachment on an electric mixer. If using an electric mixer, start slowly and gradually increase speed to medium-high. You're done when you pull out the whisk or beater and a soft peak is formed, but immediately collapses. Gently fold the egg whites into the batter and divide the batter evenly between the two prepared pans.
• Bake, rotating the pans halfway through, until the tops are golden and a toothpick inserted into the center of each cake pulls out with dry crumbs rather than wet batter, about 25 minutes. Let the cakes cool completely in the pans.
• When the cakes are completely cool and you're ready to assemble, make the frosting: Melt the butter in a medium saucepan over low heat. Stir in the brown sugar and cook, stirring constantly, for 2 minutes. Stir in the milk, raise the heat to medium-high and cook, stirring constantly until the mixture boils. Remove from the heat and let cool to lukewarm. Gradually whisk in 1 3/4 cups powdered sugar, beating until smooth. Add the additional 1/4 cup powdered sugar if the frosting is too loose. Use the frosting immediately, as it will begin to thicken and stiffen as it sits.
• To remove the cake from the pans, invert one cake pan on a serving plate, lift pan, and peel the parchment. Repeat for the second cake pan. Put one layer of the cake on a serving platter and spread about one-third of the frosting evenly over the top. Set the other layer on top, and spread the remaining frosting over the top and sides of the cake.