Combine the flours, xanthan gum, and salt in the large mixing bowl of a stand mixer and blend on low speed.

In a small bowl combine the sugar and yeast. Slowly pour in the warm water while stirring. Wait until the mixture foams slightly, then blend into the dry ingredients.

Add the butter, egg whites, and vinegar and beat on high for 3 minutes.

To form loaves, spoon dough onto greased and cornmeal-dusted cookie sheets in two long French-loaf shapes or spoon into special French-bread pans.

Slash diagonally every few inches. If desired, brush with melted butter.

Cover the dough and let rise in a warm place until doubled in size, 20 to 25 minutes.

Preheat oven to 400 degrees. Bake for 30 to 35 minutes or until golden brown.

Remove from pan to cool.