Place rack in center of oven. Preheat to 350 F degrees. Line the bottom of a springform, grease the sides. Set aside.
In a large bowl use an electric mixer to beat the bananas until smooth. Add the oil, egg yolks, water, vanilla, and pureed bananas. Beat until smooth.
Whisk together the dry ingredients (GF flour, sugar, baking powder, baking soda, cinnamon, and salt until well combined. Slowly add the dry ingredients to the banana mixture and beat until smooth, about 2 minutes.
In a separate bowl, beat egg whites and cream of tartar together in a bowl using an electric mixer until soft peaks form. Continue beating until stiff and shiny peaks form. About 6 minutes total.
Using a rubber spatula, gently fold together the banana and flour mixture into the egg white mixture. Do not over stir or it will release too much air resulting in a dense cake.
Pour batter into prepared baking pan. Smooth out top. Bake 45 to 50 minutes or until a toothpick comes out clean. Be sure to turn the cake halfway through for even browning.
Place cake on wire rack. Let cool completely in the pan, 1-2 hours. Loosen the edges with a knife and turn out the cake onto a platter. Frost the top and sprinkle on walnuts.
Dairy Free Vanilla Buttercream
Add butter and half the powdered sugar to the bowl of a stand mixer equipped with the paddle attachment. Mix on low until completely incorporated. Slowly add the second half until completely incorporated.
Add the non-dairy milk, vanilla, and salt. Mix on low until the frosting is smooth.
Frosting can be made while the cake is cooking. Store covered in the fridge until ready to use.
Do not mix the frosting on high. Low and slow is the best way to avoid gritty frosting.