Step 1: 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.
Step 2: Mix egg whites, oil and vinegar in medium bowl. Add to mixer bowl. Mix on high for 4 minutes.
Step 3: Oil tart pans. Dust with cornmeal. Ladle batter into pans. Spray buns with cooking spray.
Step 4: With cool, wet spatula, smooth tops of buns. Cover and let rise to double in warm place. (Top of stove as oven heats.)
Step 5: Sprinkle with sesame or poppy seeds.
Step 6: Bake until internal temperature is 200 degrees, about 30 minutes.
Step 7: Makes approximately 12 buns, depending on size of tart pan (these were four inches in diameter).