Preheat the oven to 350 degrees. Spray an 8X8-inch cake pan with nonstick cooking spray or line with parchment paper
In a mixer fitted with the paddle attachment, cream the butter and sugar. Next, add the egg yolks in one at a time until combined. Add in the mashed bananas, sour cream, and vanilla extract. Continue to mix until combined.
In a large bowl combine the dry ingredients (flour, baking soda, and salt). Take the flour mixture and in thirds, add it to the butter mixture until well combined. In a separate bowl add the egg whites. Using an electric mixer whip the egg whites on low speed until stiff peaks form about 3 to 4 minutes.
Take the egg whites and gently fold them into the cake batter. Pour the batter into the prepared pan and bake for about 20 minutes or until a toothpick inserted in the center comes out clean. Let cool slightly before serving for best results.
While the cake is cooling, prepare the frosting. In a stand mixer fitted with a paddle attachment, beat the unsalted butter and peanut butter until creamy. Add in the vanilla extract, kosher salt, and powdered sugar. Keep mixing until well combined and creamy.
Once the cake has cooled, frost it with the salted peanut butter frosting and enjoy!