In a bowl, beat together to blend 1 1/2 teaspoons butter (if using cooked rice flour), 1 large egg, 1/2 cup nonfat or low-fat milk, 1/2 cup sweet or cooked rice flour, and 1/2 teaspoon salt (optional). At once, fill 8 well-buttered muffin cups (2 1/2 in.) equally with batter. Bake in a 375° oven until popovers are well browned, about 35 minutes (about 30 minutes in a convection oven). Run a knife between pan and popovers and ease out of pan. Serve hot.