Heat the oven to 350°F. Lightly grease an 8x8-inch baking pan with nonstick cooking spray, line the pan with parchment paper, and grease the parchment.
In a medium bowl, whisk the rice flour, all-purpose flour, baking powder, baking soda, and salt together.
In a large bowl, mash the banana with a fork; some small chunks are fine. Add both sugars and whisk until combined.
Add the butter, sour cream, egg, and vanilla to the banana mixture and whisk until completely smooth.
Sift the dry ingredients over the wet ingredients, then whisk just until no flour lumps remain. Try not to overmix.
Pour the batter into the prepared pan and smooth the surface with a small offset spatula. Sprinkle top with chopped nuts and/or shredded coconut, or any topping you’d like. Toffee chips or chocolate chips come to mind, but that's just me :)
Bake for 25 to 27 minutes, rotating the pan halfway through, until a toothpick inserted in the center comes out clean.
Cool in the pan for 15 minutes, then transfer to a rack to cool completely before cutting into squares. This will keep for 2 to 3 days in an airtight container.