In the bowl of a stand mix, add the butter, oil, and white and brown sugars. Mix on medium speed for 1 minute to fully combine, scraping down the bowl occasionally.
To the mixer bowl, add the eggs, mashed banana and vanilla. Mix on low speed for 2 minutes.
In a medium bowl, add the sorghum flour, brown rice flour, potato starch, tapioca starch, xanthan gum, baking soda, baking powder, cinnamon and salt. Mix well.
All at once, add the flour mixture to the egg-banana mixture and mix for 30 seconds, scraping down the bowl to fully mix the ingredients. Mix on medium speed for 3 minutes.
Set batter aside to rest for 30 minutes. Meanwhile, prepare 9" x 13" baking pan by buttering well.
Preheat oven to 350 F.
Spread the batter in the prepared pan, smoothing the top.
Bake at 350° for 30 – 35 minutes or until a toothpick tests done.
Cool in the pan while preparing frosting.
For frosting:
Mix cream cheese and butter in a mixing bowl until fully combined.
Add the vanilla and continue to mix.
Add 1 cup of powdered sugar and mix.
Add another cup of powdered sugar and mix, scraping down the bowl.
Add another cup and mix again.
Add remaining powdered sugar, a little at a time until you get to the desired spreading consistency.
Frost over cooled cake.
Video
Notes
If you'd like to make cupcakes, portion batter into muffin tins and bake for 20 - 24 minutes.