Preheat your oven to 350. Grease and flour an 8" square or round pan. Mash your bananas and set aside. In a large bowl, whisk together flour, cornstarch, turbinado sugar, baking powder, baking soda, salt, and xanthan gum. Add water, oil, vinegar, vanilla, and banana extract. Stir well with a whisk until batter is "smooth" looking. Pour into the prepared pan and bake for 50 minutes. Remove from your oven and let cool for 10 minutes. Remove from the pan and let cool on a cooling rack.
Double this recipe for a 9" X 13" cake and bake for 50-60 minutes.
Optional: You can add 1/2 cup of chopped walnuts.