Heat oven to 350 degrees.Mix sugar, water and yeast in small bowl. Mix salt, xanthan gum, flours on low, in mixer. When yeast mixture is foamy, add to flours.
Mix egg whites, oil and vinegar in medium bowl. Add to mixer bowl. Mix on high for 4 minutes.
Oil tart pans. Dust with cornmeal. Ladle batter into pans. Spray buns with cooking spray.
With cool, wet spatula, smooth tops of buns. Cover and let rise to double in warm place. (Top of stove as oven heats.)
Sprinkle with sesame or poppy seeds.
Bake until internal temperature is 200 degrees, about 30 minutes.
Makes approximately 12 buns, depending on size of tart pan (these were four inches in diameter).