* 1-1/4 cups superfine rice flour, plus extra for dusting pan * 1/2 cup potato starch * 1/3 cup tapioca starch * 1-1/2 teaspoons baking powder * 1/2 teaspoon salt * 1/2 teaspoon xanthan gum * 3/4 cups (1-1/2 sticks) butter, at room temperature * 1 cu