Preheat oven to 350 degrees. Line a 12-cup
muffin tin with baking cups. Whisk
together brown-rice flour, ground walnuts,
potato starch, tapioca flour, baking
powder, xanthan gum, and salt.
Whisk together yolks, oil, sugar, bananas,
and vanilla. Fold flour mixture into banana mixture.
Whisk whites until stiff peaks form.
Working in 3 batches, fold whites into
batter. Fill each baking cup with batter.
Bake until light golden brown and
a toothpick inserted into the centers
comes out clean, 22 to 24 minutes. Let
cool slightly. Turn out muffins onto a wire rack, and let cool completely.