Preheat oven to 350 degrees. Spray or butter a 10 x 15 inch jelly roll pan.
In a bowl mash bananas and set aside. This can be done with mixer or by hand.
In a bowl sift together flour, baking powder, baking soda, and salt and set aside.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until light and creamy (about 1 minute). Add both sugars and beat on high speed for 2 minutes until creamed together. Scrape down the sides and bottom of the bowl with a spatula as needed. Add the eggs and the vanilla. Beat on medium-high speed until combined. Then beat in the mashed bananas. Scrape down the bowl as needed.
With the mixer on low speed, add the dry ingredients in three additions alternating with the buttermilk and mixing each addition just until incorporated. Do not overmix. The batter is slightly lumpy and thick. This is okay!
Pour the batter into the prepared pan and spread out evenly. Bake for 18 to 25 minutes. Baking times vary, so keep an eye on yours. The cake is done when a toothpick inserted in the center comes out clean. If you find the top of the cake is browning too quickly in the oven, loosely cover it with aluminum foil.
Remove cake from oven and let cool completely on a wire rack. (After it is mostly cooled you can put in refrigerator or freezer to speed up process).
Whipped Cream Cheese Icing
In the bowl of a stand mixer (or hand-held mixer), beat very cold whipping cream on medium-high until stiff peaks form; about 2-3 minutes.
In clean stand mixer bowl (or using a hand-held mixer) whip cream cheese until light and smooth. Add in sugar, salt, and vanilla and beat until fully incorporated.
Remove bowl from stand mixer and gently fold whipped cream into cream cheese mixture.
Serve with sliced bananas dipped in lemon juice and garnished with chopped walnuts if desired.
Cover leftover cake tightly and store in the refrigerator for up to 5 days.