Preheat the oven to 400°F. Grease a 12-cup popover pan or standard muffin pan., Popover batter is easiest made in a blender. Blend eggs, butter, and milk on medium speed in the blender until the mixture is uniform. Whisk the flour or flour blend in a bowl with the xanthan gum and salt. Spoon or pour the dry ingredients into the blender, then blend at medium-high speed until batter is smooth. A few tiny lumps are OK, but if you have more lumps than that, keep blending till they disappear., If you don't have a blender, whisk together the eggs, butter, and milk in a large bowl. In a separate bowl, whisk the flour or flour blend with the xanthan gum and salt, then gradually sift and whisk into the liquid ingredients until you have a smooth batter. If, despite your best efforts, there are more than a few tiny lumps, strain them out of your batter with a sieve., Pour the batter into the prepared pan, filling each cup about 2/3 full., Bake for 25 minutes, then reduce the oven heat to 350°F and bake for an additional 15 minutes, until the popovers are deep brown., Remove from the oven, prick the side of each popover with a sharp knife to let the steam out, and let popovers rest for 5 minutes to finish setting. Remove from the pan, and serve immediately. Or, to keep them crisp longer, allow them to sit in the turned-off oven for 10 to 15 minutes before serving.