Preheat your oven to 325°F. Grease or line a 9-inch square baking dish and set it aside.
Make the cake
In a small bowl, place the flour blend, xanthan gum, baking soda, baking powder and salt, and whisk to combine well. Set the bowl aside.
In a large bowl, place the 6 tablespoons of (5 if the lighter version) butter, granulated sugar, and brown sugar, and beat well with a hand mixer until smooth and light.
Add the eggs (or egg and egg whites in the lighter version), and beat until smooth.
Add the vanilla and mashed bananas and mix with a spoon or spatula until just combined.
Add the milk, and mix until just combined.
Add the dry ingredients to the mixing bowl, and mix until just combined. The batter will be thick but not stiff.
Scrape the cake batter into the prepared pan, and smooth into an even layer with a wet spatula.
Place the pan in the center of the preheated oven and bake for 22 to 24 minutes, or until the cake springs back when pressed gently in the center.
Remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
Make the frosting
Place all the frosting ingredients in a large bowl (or the clean bowl of your stand mixer fitted with the paddle attachment) and beat with a hand mixer until smooth (about 3 minutes).
Spread the frosting on top of the cooled cake, then slice the cake with a sharp knife into 9 equal pieces. Serve immediately.
Video
Notes
Flour blends.My favorite gluten free flour blends are Better Batter's original blend gluten free flour and Nicole's Best multipurpose blend. Bob's Red Mill 1-to-1 Gluten Free Baking Flour should also work, but you'll need to add an additional 1/4 teaspoon xanthan gum to the cake dry ingredients or it will be crumbly.Cup4Cup changed its formula and doesn't seem to work as well as it has in the past, so I don't recommend it. To make your own blend using one of my “mock” recipes, please see the all purpose gluten free flour blends page.Nutrition information.Nutritional information is for the "regular" version of the cake, not the lighter version.