Gluten-Free Banana Cake (Dairy-Free)

Place the rack in the center of the oven and preheat to 350°F. Line the bottom of a springform pan with parchment paper and 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.
In a separate bowl, whisk together the dry ingredients: gluten-free flour, sugar, baking powder, baking soda, cinnamon, and salt. Slowly add the dry mixture to the banana mixture and beat until smooth, about 2 minutes.
In another clean bowl, beat the egg whites and cream of tartar using an electric mixer until soft peaks form. Continue beating until stiff, shiny peaks form—about 6 minutes total.
Using a rubber spatula, gently fold the banana-flour mixture into the egg white mixture. Do not overmix, as this can deflate the batter and result in a dense cake.
Pour the batter into the prepared pan and smooth the top. Bake for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Rotate the cake halfway through baking for even browning.
Place the cake on a wire rack and let it cool completely in the pan, about 1–2 hours. Loosen the edges with a knife and carefully turn the cake out onto a platter. Frost the top and sprinkle with walnuts.

Add the butter and half of the powdered sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed until fully incorporated. Gradually add the remaining powdered sugar and mix until incorporated.
Add the non-dairy milk, vanilla, and salt. Mix on low speed until the frosting is smooth and creamy.