This Banana Chiffon Cake is light, fluffy, and has just the right amount of sweetness. It's full of banana flavor yet supremely tender. Be sure to use a tube pan with removable bottom, and don't oil that pan.
Preheat oven to 325F, with rack on lower middle position.
In a large bowl, whisk together the flour, sugar, baking powder, and salt until well combined. Make a well in the center. Add oil, unbeaten yolks, water, vanilla, and mashed bananas. Beat until smooth, about 2 minutes.
In a separate bowl, beat together the egg whites and cream of tartar on medium-high speed until stiff peaks form, about 4 min. Be sure it's at stiff peaks, not soft peaks.
Using a rubber spatula, gently fold together the banana mixture into the egg white mixture. Carefully fold repeatedly until batter is incorporated, but do not over-handle the batter.
Pour batter unto an ungreased tube pan with removable center/bottom. Bake 60 minutes or just until toothpick inserted in center comes out almost clean (a few small tender crumbs attached is perfect.)
Invert the pan over a plate; keep pan inverted until cake is completely cool. Once cooled, use a very thin knife and run it carefully around all edges of the cake to loosen cake. Turn upside down onto a plate, and carefully remove the bottom/center of pan from the cake.